Median Cross-Section Design for Rural Divided Highways (2014)

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5 C H A P T E R 2 2.1 AASHTO Median Design Guidelines The AASHTO Roadside Design Guide (2) and the AASHTO Policy on Geometric Design of Highways and Streets (com- monly referred to as the Green Book) (1) contain guidelines for the design of medians on divided highways. Included are guidelines related to median width, median cross-slopes, and median barrier warrant and placement criteria. This litera- ture review examines and synthesizes existing median design policies and research related to them. The most recent update to the AASHTO Roadside Design Guide (2) was made in 2006. This update references the per- formance requirements for median barriers and contains guidelines for selecting and installing an appropriate bar- rier system. Characteristics of median barrier systems are included in this update. This section describes the guidance outlined by AASHTO policies regarding median width, median side slopes, approved median barriers, and median barrier placement guidelines. 2.1.1 Median Width The median width is a linear dimension between the edges of the traveled way on divided highways, including the left shoulders. Functionally, medians are intended to separate opposing traffic, provide a space for emergency stopping, pro- vide a recovery area for out-of-control vehicles, allow space for speed changes and storage of left-turning and U-turning vehi- cles, minimize headlight glare, and provide width for future lanes (1). General guidance suggests that median widths should range between 1.2 to 24 m (4 to 80 ft). Depressed medians are generally suggested on freeways. Widths greater than 12 m (40 ft) are intended to provide drivers with a sense of separation from traffic traveling in the opposing lanes. Median widths between 15 to 31 m (50 to 100 ft) are com- mon on rural freeways. Such a dimension is easily achievable in areas with level terrain with no right-of-way restrictions and where alignments are often parallel. In rural areas with rolling terrain, independent vertical profiles commonly are used to blend the freeway into the environment. Again, wide median widths are achievable. Narrow median widths (3 to 9.1 m [10 to 30 ft]) may be needed in mountainous terrain or where right-of-way restrictions dictate. In certain instances, the median width guidelines set forth in the AASHTO Green Book (1) may not be obtainable. Alter- natively, cross-median crashes may occur frequently although the design guidelines are adhered to closely. In either case, median barriers are used to prevent cross-median crashes at narrow median sites. Median barrier warrant criteria are pro- vided in the previous edition of the Roadside Design Guide (3) and are considered for application based on combinations of median width and average daily traffic volumes. Figure 2-1 shows these criteria. For median widths up to 9 m (30 ft) and traffic volumes greater than 20,000 vehicles per day, median barrier is typically evaluated. Between 9 to 15 m (30 to 50 ft), regardless of average daily traffic (ADT) volumes, median barrier is considered optional. For medians greater than 15 m (50 ft), median barrier is not normally considered (2). The median barrier warrant criteria shown in Figure 2-1 are for high-speed, controlled-access highways that have tra- versable median slopes. In the “Barrier Optional” region of Figure 2-1, a cross-median crash problem may dictate the need for median barrier. The guidelines for median barriers for high-speed, fully controlled-access roadways were updated in the new edition of the AASHTO Roadside Design Guide (2) in 2006 and are shown in Figure 2-2. These revised guidelines recommend median barrier on high-speed, fully controlled-access road- ways when the median is 9 m (30 ft) in width or less and the average daily traffic is greater than 20,000 vehicles per day. For locations with median widths less than 15 m (50 ft) and where the ADT is less than 20,000 vehicles per day, a median barrier is optional. For locations where median widths are Literature Review

6greater than 9 m (30 ft) but less than 15 m (50 ft) and where ADT is greater than 20,000 vehicles per day, a median barrier should be considered. Studies in determining a need for median barrier in these circumstances can include cost/benefit analysis or an engineering study evaluation considering such factors as traffic volumes, vehicle classifications, median crossover his- tory, crash incidents, vertical and horizontal alignment rela- tionships, and median/terrain configurations. Where median widths are greater than 15 m (50 ft) a barrier is not normally considered except in special circumstances such as a location with significant history of cross-median crashes. 2.1.2 Median Side Slopes Median slopes are designed to provide adequate drainage channels to convey storm run-off between opposing direc- tions of travel, and to provide a traversable recovery area for errant vehicles that leave the roadway to the left of the travel lanes. To accomplish these objectives, the Green Book (1) recommends 1V:6H side slopes. Steeper slopes (e.g., 1V:4H) may be adequate. Slopes flatter than 1V:6H are often required when placing longitudinal median barrier on a slope. The cable median barrier, however, is effective on 1V:6H slopes and, some manufacturers claim, even on slopes steeper than 1V:6H. 2.1.3 Median Barrier Types Longitudinal median barriers may be rigid, semi-rigid, or flexible. Rigidity is measured in terms of the barrier’s design deflection distance as determined in a standardized vehicle impact test. Longitudinal median barrier systems approved in the AASHTO Roadside Design Guide, Chap- ter 6—Update (2), and their test levels, are shown below: • Weak-post, W-beam guardrail TL-3 • 3-strand cable, weak post TL-4 • High-tension cable barrier TL-3 or TL-4 • Box-beam barrier TL-3 • Blocked-out W-beam (strong post) TL-3 or TL-2 • Blocked-out thrie-beam (strong post) TL-3 • Modified thrie-beam TL-4 • Concrete barrier TL-4 or TL-5 • Quickchange moveable barrier TL-3 Generally, flexible median barrier systems have lower instal- lation costs than semi-rigid or rigid systems. Flexible systems usually require greater maintenance costs than more rigid sys- tems. Also, the impact forces associated with rigid barriers are much greater on the impacting vehicle than those associated with flexible barriers. The 2006 edition of the AASHTO Roadside Design Guide (2) reports that there are currently five high-tension cable barrier systems. Characteristics of these barriers systems are shown in Table 2-1. Each of these systems is proprietary and utilizes a unique post design. 2.1.4 Median Barrier Placement Guidelines In level terrain, symmetric medians are commonplace. In rolling or mountainous terrain, however, asymmetric medi- ans may be constructed due to topography or environmental constraints. Guidelines for placing median barrier in these cross sections are provided in the AASHTO Roadside Design Guide (2) and shown in Figure 2-3. The dimensions in Figure 2-3 are as follows: • W = median width (ft or m) • W/2 = one-half the median width (ft or m) • S2 = left median side slope • S3 = right median side slope • a, b, c, d, e = median barrier placement locations Section I of Figure 2-3 shows guidelines for depressed medians; Section II shows placement illustrations for medians with significant traveled way elevation differences; Section III illustrates raised median barrier applications. Figure 2-1. Median barrier warrant criteria from 2002 AASHTO Roadside Design Guide (3).

7 Figure 2-2. Guidelines for median barriers on high-speed, fully controlled-access roadways from 2006 AASHTO Roadside Design Guide (2). Cable Barrier Characteristics Cable barrier name No. of cables Cable heights Date of earliest installation Crash tests Dynamic deflection Slope requirement* Standarda 3 Top: 30 in Middle: 26 in Bottom: 21 in TL-4 11.5 ft up to 1V:6H Brifenb 4 Top: 36.5 in 3rd: 30.5 in 2nd: 25.0 in 1st: 19.0 in 2000 TL-4 Small car: 4.25 ft Pickup: 7.25 ft up to 1V:6H Trinity CASS 3 Top: 29.5 in Middle: 25.0 in Bottom: 21.0 in 2003 TL-4 Pickup: 7.7 ft up to 1V:6H Gibraltar 3 Top: 39 in Middle: 30 in Bottom: 20 in 2005 TL-4 Car (Geo Metro): 2.5 ft Truck (GMC Sierra): 8.6 ft up to 1V:6H Safence 4 Top: 36.5 in 3rd: 30.5 in 2nd: 25.0 in 1st: 19.0 in TL-3 Small car: 3.7 ft Pickup: 5.9 ft up to 1V:6H U.S. High- Tension Cable System 3 Top of post: 33.0 in Top cable: 29.5 in Middle cable: 25.5 in Bottom cable: 21.5 in 2002 TL-3 Pickup: 6.5 ft up to 1V:6H a As in the Roadside Design Guide. b Measurements only found for TL-4 traffic barrier. *Note: No documentation on 1V:4H slope. Table 2-1. Characteristics of high-tension cable barrier systems.

8A roadside barrier may be required either to prevent errant vehicles from colliding with a fixed object in the median or to prevent vehicles from overturning when traversing the slope. Placement locations “b” and “d” in Figure 2-3 are at the edge of the inside (or median) shoulder and are intended to prevent errant vehicles from encroaching onto the median slope. Reasons for placing barriers at such locations vary; however, the most common reasons are that fixed objects are located on the slope or the median slope(s) are not tra- versable. Placement location “a” is at or near the center of the median—barriers can be placed at such a location when the risk of a vehicle overturning is low. Median barriers per- form best when the impacting vehicle has all wheels on the ground. 2.2 Review of Median Safety Studies Past research on median safety has investigated either the factors that caused vehicle encroachments or median crash frequency or severity. Additionally, early cross-median crash analysis was performed using simulation models. The follow- ing sections summarize the history of median safety research by reviewing past encroachment and crash studies, as well as simulation studies. Research related to median barrier type and side slope design also is included in the following sections. Last, several state transportation agencies use median barrier warrants that are not the same as those recommended in the AASHTO Roadside Design Guide (2). The research or engi- neering studies used to establish the warrants are presented. Figure 2-3. Median barrier placement guidelines (2).

9 2.2.1 Encroachment Studies Early median safety studies sought to determine and quan- tify factors that caused vehicle encroachments into the median area on divided highways. In the early 1960s, Hutchinson and Kennedy (4) studied vehicle encroachments along I-74 and the Kingery Expressway (I-57) in Illinois. Each facility was a four-lane divided highway. I-74 had a depressed median width of 12 m (40 ft) while the Kingery Expressway had a depressed median width of only 5.5 m (18 ft). After 6 years of data collection, four relationships were observed, each con- taining ADT as one-half of the relation. One of the relation- ships examined was ADT versus encroachment rate, which is based on encroachments per 100 million vehicle-miles trav- eled. It was shown that for ADT volumes of 4,000 vehicles per day and less, the encroachment rate was stable and slightly above 400 per 100 million vehicle-miles traveled. As the ADT increased from 4,000 to 5,000 vehicles per day, there was a sharp decline in the encroachment rate to approximately 150 encroachments per 100 million vehicle-miles traveled. As the ADT volume continued to increase, the encroachment rate then stayed relatively constant at 150 encroachments per 100 million vehicle-miles traveled. Figure 2-4 shows the rela- tionship between ADT and encroachment rates. The driving environment was considered a primary reason for the fluctuation in encroachment rates in relation to traf- fic volumes. At low traffic volumes, drivers are less attentive. There is more freedom of movement within the travel lanes and the only restrictions are the physical features of the road- way (4). Therefore, it is likely that vehicles tend to sway off the traveled way and eventually into the median area. As traffic volumes increase, driver alertness also increases and the per- centage of “lateral veering” vehicles is greatly reduced because of the decreased vehicle spacing within the traffic stream. In addition, with the presence of other vehicles, a “follow-the- leader” phenomenon results in which vehicles farther back in the traffic stream tend to position in the same vehicle path as those farther downstream. Another relationship studied by Hutchinson and Kennedy related ADT to the average encroachment angle. As ADT increased from 2,000 to 6,000 vehicles per day, the encroach- ment angle also increased from 9 to 14 degrees. Figure 2-5 shows the relation between ADT and average encroachment angle. The theory behind these observations is that there is an increase in vehicle conflict as traffic volumes increase. As a result, a driver may suddenly leave the traveled way and Figure 2-4. Encroachment rate for Interstate 74 and Kingery Expressway (4). Figure 2-5. Relationship between ADT and average encroachment angle (4).

10 enter the median area because another vehicle unexpectedly merges into the occupied lane. Hutchinson and Kennedy also examined the relationship between ADT and the percent of vehicles that crossed into the median. As shown in Figure 2-6, as ADT increased from 4,000 to 6,000 vehicles per day, the percentage of vehicles crossing into the median increased. The final relationship studied by Hutchinson and Kennedy was that between ADT and lateral distance traveled by encroach- ing vehicles on I-74. The ADT volumes range from 2,000 to 6,000 vehicles per day. These vehicles traveled an average of 5.8 to 8.2 m (19 to 27 ft) into the median area over this range of ADTs. This relationship is shown in Figure 2-7. Beginning in 1976, single-vehicle run-off-the-road acci- dents on both multilane divided and undivided rural highways were studied in Canada (5). Through 1978, a 9-km (5.6-mi) section of an urban freeway in Ottawa was studied that had ADT volumes ranging from 50,000 to 100,000 vehicles per day. With the exception of a 1-m (3-ft) paved shoulder, the median was grass. Over the 2-year study period, 140 encroach- ments were observed and the results were very similar to those in other studies. The following is a list of the study results obtained by Sanderson: • Average roadway departure angle for both median and right-side encroachments was 14 degrees. • Median encroachments were twice as many as right-side encroachments. • There was a significant disparity between the numbers of encroachments reported to those observed. The ratio between observed and reported median encroachments was 3 to 1, while the ratio for right-side encroachments was 4 to 1. In 1978, data from several Canadian provinces were col- lected to investigate single-vehicle run-off-the-road acci- dents on divided and undivided rural highways. A multiple regression analysis of 1,937 encroachments explained only 30 percent of the variance between accidents and traffic volumes (5). Factors such as alcohol, weather, and driver variables were considered to have a significant effect on the models developed. This study showed no significant correla- tion between ADT volumes and encroachment rates; how- ever, when the data were forced into 2,000 vehicle per day groupings and averaged over a set of ranges, the results were nearly identical to the Hutchinson and Kennedy (4) study discussed previously. This study also showed that, on aver- age, the ratio of observed-to-reported accidents was 3.75 to 1 for two-lane undivided highways and 5 to 1 for multilane divided highways. 2.2.2 Crash Studies Crosby (6) evaluated the cross-median crash experience on the New Jersey Turnpike over a 7-year period (1952 through 1958, inclusive). The accident data were from the original 190-km (118-mi) New Jersey Turnpike before the installa- tion of median guardrails. In 1958, 29 km (18 mi) of median guardrail were installed on sections with variable median widths (1.8 to 7.9 m [6 to 26 ft]). The data used for the research were limited to the through travel lanes (excluding those within service areas), interchanges, and their intercon- necting roadways and ramps. During the analysis period, 48 of 158 (30.4 percent) fatal crashes were considered cross- median crashes. During the analysis period, there were a total of 455 cross-median crashes. They constituted approximately 8.3 percent of all crashes on the New Jersey Turnpike dur- ing the analysis period (455 of 5,473 total collisions). The cross-median crash rate was higher when the medians were narrower. Garner and Deen (7) compared various median types on divided, four-lane Interstate highways with similar geomet- ric features in the state of Kentucky. The two variables that Figure 2-6. Relationship between ADT and percent vehicles crossing into median (4). Figure 2-7. Relationship between ADT and encroachment distance (4).

11 were the primary focus in the study were median width and median cross section. For the routes studied, variables such as pavement width and shoulder width remained constant. The types of medians analyzed in the study were raised, depressed, deeply depressed, and irregular medians. The results of the Garner and Deen study verified previ- ous conclusions from other researchers that wider medians are safer. Their data indicated that the percentage of vehicles crossing the median decreases as the median width increases. Their data also indicated that the relationship between acci- dent rate and median width was not clear. However, deeply depressed medians had a higher accident rate than raised medians. Garner and Deen suggested that the beneficial effects of wide medians can be offset by steep median side slopes. As such, they recommended slopes of 1V:6H or flatter when the median is 18 m (60 ft) wide. Additionally, median widths of 9 to 12 m (30 to 40 ft) were recommended on high-speed divided highways. However, Garner and Deen also stated that other median elements, such as cross slopes, and the presence of obstructions can have a greater effect on median safety than the width. Median cross-slopes play a major role in the safety aspects of a median. Deeply depressed medians that have cross slopes of 1V:4H and 1V:3H for an 11-m (36-ft) wide median have been shown to have a significantly higher accident rate than the raised medians for widths of 6, 9, and 18 m (20, 30, and 60 ft). Medians with steep slopes do not provide reasonable recovery areas and are often hazards in themselves (7). In addition, steep slopes also increase the likelihood of vehicle rollover. It was shown that the alignments studied with cross slopes of 1V:4H and 1V:3H had 10.3 and 16.5 accidents per 100 million vehicle-kilometers (6.4 and 10.3 accidents per 100 million vehicle-miles) of travel, respectively, but the average accident rates for the alignments with other types of medians were averaged to be 2.08 accidents per 100 million vehicle-kilometers (3.35 accidents per 100 million miles). The raised median design analyzed in the Garner and Deen study also was shown to have some downfalls. This design seemed to have a higher number of crossover crashes. It was concluded that when drivers hit the median, they tend to overreact, which causes them to lose control of the vehicle. There also are disadvantages associated with raised medians. Raised medians do not provide an adequate storage area for snow removal. Also, water tends to migrate onto the roadway, which allows icy spots to form during cold weather. Garner and Deen also concluded that irregular medians, which have a varying median width and nature, have higher median acci- dent rates, total accident rates, and severity rates. Foody and Culp (8) studied the safety aspects between raised and depressed medians, each having a 2.6-m (84-ft) design width. They observed the accident frequency and severity of single-vehicle accidents of four-lane divided Inter- states in Ohio from 1969 to 1971 for each median type. They observed 201 km (125 mi) of highway with the raised median design and 166 km (103 mi) of highway with the depressed median design. The depressed median had side slopes that are 1V:8H. The raised median had 1V:8H foreslopes and 1V:3H backslopes. The study detailed single-vehicle median accidents, accident severity, vehicle path encroachments, and median rollover accidents. The following summarizes the study results obtained by Foody and Culp: • The accident rate was slightly higher for the raised median than for the depressed median section. • There was no difference in injury-related accidents between the two median types. • There was no difference between the two types of medians in the number of median encroachments. • There was no significance in the difference of rollover fre- quency between the two median designs. A study by Kniuman et al. (9) investigated median safety using Highway Safety Information System (HSIS) data from Utah and Illinois. In Utah, the total accident rate was found to decline from 404 accidents per 100 million vehicle-kilometers (650 accidents per 100 million vehicle-miles) for medians with zero width to 69 accidents per 100 million vehicle-kilometers (111 accidents per 100 million vehicle-miles) for median widths in the range of 26 to 34 m (85 to 110 ft). In Illinois, the data suggest a similar trend, with an accident rate of 430 acci- dents per 100 million vehicle-kilometers (692 accidents per 100 million vehicle-miles) for medians with zero width and 33 accidents per 100 million vehicle-kilometers (53 accidents per 100 million vehicle-miles) where the median is 26 to 34 m (85 to 110 ft) in width. It was also reported that the average rate of head-on collisions for median widths greater than 17 m (55 ft) was 0.6 and 1.9 accidents per 100 million vehicle- kilometers (1 and 3 accidents per 100 million vehicle-miles) for the Utah and Illinois data, respectively. For the Utah data, single-vehicle accidents do not decline as the median width is increased from a range between 0.3 to 7 m (1 to 24 ft) to a range between 26 and 34 m (85 and 110 ft). For the Illinois data, single-vehicle accidents were found to decline by almost half as the median width increased from a range of 0.3 to 7 m (1 to 24 ft) to a range of 26 to 34 m (85 to 110 ft). It was also shown that little reduction in accident rate was obtained for median widths in the range of from 0 to 8 m (0 to 25 ft). The most apparent decline in total accident rate was found to occur roughly between 6 and 9 m (20 and 30 ft). For medians between 18 and 24 m (60 and 80 ft), the decline in accident rates seems to level off. All of the previously discussed results can be found in Table 2-2, which is an excerpt from the study results.

12 Relationships were developed between the type of collision and the relative effects of the median width. The type of acci- dent most affected by the increase in median width was head- on collisions. For the Utah data, the relative effects were fairly linear. There was an approximate 17 percent decrease in the relative effect of increasing the median width in 3-m (10-ft) increments. For the Illinois data, there was a sharp decline in the relative effect between median widths in the range from 3 to 12 m (10 to 40 ft). The largest decline was in the interval from 3 to 6 m (10 to 20 ft), in which there was a 45 percent decrease in the relative effect of increasing the median width. From 6 to 12 m (20 to 40 ft), the average decline in relative effect of median width was 42 percent. For median widths greater than 12 m (40 ft), the relative effect of increasing the median width for head-on collisions stayed fairly constant around 0.10, equivalent to a 10 percent reduction in the total accident rate. The validity of the Kniuman results observed from the Illinois and Utah HSIS study is controlled by the variables used. Other variables were either not measured by the data- base or not used in the final model simply because of the need to limit the model to as few variables as possible (9). Other variables that could have been included in the model were median slope, type of traffic, environmental factors, and other geometric factors. The general results of this study indi- cate that crash rates decrease as the median width increases. It also is apparent from the data that there is little decrease in crash rate for medians less than 6 to 9 m (20 to 30 ft). There- fore, increases in safety effects are not seen until the median reaches at least 6 to 9 m (20 to 30 ft) in width. Even larger safety benefits can be seen for median widths up to 20 to 24 m (65 to 80 ft), at which point the safety effects of increasing median width begin to level off. Mason et al. (10) recently used crash and roadway inven- tory data to characterize CMCs on Pennsylvania Interstates and expressways. In 5 years, 267 of these crashes occurred where 15 percent resulted in fatalities and 72 percent resulted in injury-type crashes. When compared to all crash types on Interstates and expressways, the severity level of CMC collisions is significantly more severe. Additionally, nearly 63 percent of CMCs occurred during daylight con- ditions, 58 percent occurred during wet or snow and icy conditions, and 12 percent involved drugs or alcohol usage. Limited field data collection found that median shoulder width, roadway grade, median cross-slopes, the presence and degree of horizontal curvature, presence of roadside obstacles, and vehicle type did not statistically influence CMCs. However, there was preliminary evidence to con- clude that the presence of interchange entrance ramps does increase the likelihood of CMCs. Using the CMC data from Pennsylvania, Donnell et al. (11) estimated models of crash frequency for Interstate highways. The model took the form shown in Equation 1. 0.2 (1)18.203 1.770 0.0165N e L ADT eCMC MW= − −• • • where NCMC = number of CMCs per year for one direction of travel L = segment length (mi) ADT = average daily traffic (veh/day) MW = median width (ft) All of the parameters were statistically significant (p < 0.0001). However, the model explained only a relatively low proportion Median width (ft) Average crash rate (crashes per 100 veh-mi traveled) Category Mean N Single vehicle Head-on Rollover Total Utah 0 0.0 176 127 10 14 650 1 to 10 9.4 257 97 10 5 618 11 to 29 14.9 213 89 8 7 462 30 to 54 46.3 52 109 1 29 159 55 to 84 71.7 179 106 1 22 137 85 to 110 101.0 105 93 0 29 111 All 32.0 982 103 6 14 424 Illinois 0 0.0 567 86 21 5 692 1 to 24 12.8 199 69 12 8 647 25 to 34 29.8 176 92 3 15 292 35 to 44 39.7 479 51 2 6 129 45 to 54 49.2 200 61 2 7 127 55 to 64 63.8 450 27 1 3 45 65 to 84 71.9 239 40 1 5 59 85 to 110 88.9 171 36 1 6 53 All 39.4 2481 58 7 6 283 Table 2-2. Relationship between median width and accident rate in Utah and Illinois (9).

13 of the variation in CMC frequency. Interpretation of the ADT parameter (1.77) suggests that CMCs are a two-stage process. First, the likelihood that a vehicle loses control and enters the median when traveling in one direction of travel is roughly proportional to the traffic volume (ADT) in that direction. The out-of-control vehicle must than traverse the median, enter the opposing traveled way, and collide with a vehicle traveling in the opposite direction. The likelihood of this occurring should be roughly proportional to the one-way traffic volume (ADT) in the opposing travel lanes. Because the traffic volumes on opposing roadways are typically quite similar on Interstate highways, it seems logical that the likelihood of a CMC be roughly proportional to the square of the one-way ADT. A one unit increase in the median width decreases CMC frequency by approximately 1.7 percent. Donnell and Mason (12) used negative binomial regres- sion to predict the frequency of median barrier crashes on Pennsylvania Interstate highways. There were a total of 4,416 median barrier collisions that occurred during the 5-year study period (1994 through 1998) on 1,188 km (738 mi) of divided highway protected with a longitudinal barrier. The ADT, presence of an interchange entrance ramp, posted speed limit, horizontal curve indicator, and median barrier offset from the left-edge of the traveled way were all statistically significant predictors of median barrier crash fre- quency. Curved roadway sections were expected to increase the median barrier crash frequency, holding all other vari- ables constant. A unit increase in the median barrier offset was expected to decrease the median barrier crash frequency by 3.5 percent, holding all other variables constant. A lower posted speed limit was expected to decrease the median barrier crash frequency while the absence of interchange entrance ramp also was expected to decrease the expected median barrier crash frequency, holding all other variables constant. Donnell and Mason (13) predicted the severity of both CMCs and median barrier crashes using crash event and roadway inventory data from Pennsylvania Interstate high- ways. Three severity levels (fatal, injury, and property damage only [PDO]) were considered. In the CMC severity model, an ordered response was used while multinomial logistic regression was used to estimate median barrier crash sever- ity. In the CMC severity model, the use of drugs or alcohol and the direction of the horizontal curve influenced severity. The predicted probabilities of a fatal CMC were between 9.8 and 24.3 percent when considering the various catego- ries of independent variables. The predicted probabilities of an injury CMC were between 68.1 and 70.5 percent when considering the various categories of the independent vari- ables. The assumption of parallel regression lines was vio- lated when predicting the severity of median barrier crashes. As such, a nominal response was considered. The independent variables that influenced crash severity included pavement sur- face condition, drug or alcohol use, the presence of an inter- change ramp, and ADT. The predicted severity probabilities were as follows: • Fatal: 0.5 to 0.8 percent; • Injury: 53.5 to 60.2 percent; and • PDO: 39.0 to 46.0 percent. Donnell and Mason (14) used both CMC and median bar- rier crash frequency and severity models to evaluate existing median barrier warrant criteria in Pennsylvania. Interstate highways with and without median barrier were compared using roadway inventory and crash data. The economic eval- uation consisted of benefits derived from changes in crash costs and the costs were derived from barrier installation, maintenance, and user costs. The benefit-cost analysis results are shown in Figure 2-8. In Figure 2-8(a), the concrete barrier was assumed to be installed only in the center of a median. The gray-shaded area represents benefit-cost (B/C) ratios that exceed 1.0 and where the data used in the analysis represent most CMCs and (a) Concrete Median Barrier (b) W-Beam Guiderail Median Barrier NB = No barrier Figure 2-8. Benefit-cost ratios for median barrier installation (14).

14 median barrier crashes. The outlined region also contains B/C ratios that exceed 1.0. The frequency of crashes was very low in the outlined region and, therefore, a site-specific evalu- ation, using the methodology described, was recommended. In Figure 2-8(b), two numerical values are shown in each cell. The value on top represents the B/C ratio for center place- ment location, while the value on the bottom represents a 1.2-m (4-ft) offset from the edge of the traveled way. Because the strong post W-beam guiderail used along medians in Pennsylvania has design deflection ranging from 0.6 to 1.2 m (2 to 4 ft), they are not used when the median is less than 3 m (10 ft) wide. The “note” in Figure 2-8(b) represents a condi- tion where no benefits were found by considering a longitu- dinal barrier. Either the crash severity or frequency did not change enough when comparing the with-without median barrier scenario to show a net benefit in crash cost. Ulfarsson and Shankar (15) estimated a predictive model of median crossover crash frequencies with a multiyear panel of cross-sectional roadway data using data for the State of Washington. The study compared three different count regres- sion models, including: negative multinomial (NM), nega- tive binomial (NB), and random-effects negative binomial (RENB). The results showed that the negative multinomial model outperformed the other two due to the existence of section-specific correlation in the panel. Variables considered in the model included indicator variables for the following conditions: • ADT less than 5,000 vehicles per day; • ADT between 5,000 and 10,000 vehicles per day per lane; • Median width between 9 to 12 m (30 to 40 ft); • Number of horizontal curves per km; • Length of section (km) if median width is less than 12 m (40 ft); • Length of section (km) if median width is between 12 and 18 m (40 ft to 60 ft); • Length of section (km) if median width is greater than 18 m (60 ft); • Difference between maximum and minimum shoulder width is greater than 1.2 m (4 ft) and the number of hori- zontal curves is greater than two per section; • Roadway friction factor if number of horizontal curves is greater than 0.67 per km (1.08 per mi); and • Section located (either Interstate Route 90, Interstate Route 205, US Route 2, or State Route 16). These indicator variables had the value of 0 when the con- dition specified was not present or not applicable, and had the value of 1 (or a specified length, width, or number of curves) if the condition specified was present or applicable. A comparison of the model output for NM, NB, and RENB is shown in Table 2-3. The results of the negative multinomial regression model indicate that crash frequencies are lower along road sections with lower traffic volumes. The pre- dicted median crossover crash frequency decreases as the number of horizontal curves per km increases. However, the indicator for the difference between the maximum and minimum shoulder width (greater than 1.2 m [4 ft]) and number of horizontal curves is greater than two per sec- tion, suggesting that the crash frequency increases as the curve frequency and shoulder width difference increases. The section length variables were all positive in the negative multinomial model. Miaou et al. (16) presented predictive models of crash frequency and severity as well as B/C analysis results for a cross-sectional with-without median barrier study in Texas. Two years of data (1998 and 1999) were collected from Inter- states, freeways, and expressways with four or more lanes and a posted speed limit of 88 km/h (55 mph) or greater. Only divided highway sections with ADT less than 150,001 vehicles per day were considered in the analysis as were sections with medians between 4.6 to 45.7 m (15 to 150 ft) wide. There were 346 cross-median crashes in 52 Texas counties during the 2-year analysis period. An additional 3,064 median-related crashes were identified on sections with no longitudinal median barrier. There were 3,672 median- related crashes included in the analysis time period along sec- tions with longitudinal median barrier. Of these 3,672 crashes, 2,714 crashes (74 percent) were defined as hit-median-barrier crashes. The following four median crash types were considered in the frequency and severity models: cross-median crashes on sections with no barrier, other median-related crashes on sections with no barrier, all median-related crashes on sec- tions with a barrier, and hit-median-barrier-only crashes on sections with a barrier. A Poisson-gamma model, using a full Bayes approach, was used to specify and estimate the crash frequency prediction model. The advantage of using such a modeling technique is that it accounts for the uncertainty associated with the model parameter estimates. The roadway inventory and traffic volume variables included in the models were as follows: • Median width (ft); • Logarithm of ADT; • Number of lanes; • Posted speed limit (dummy variable for 96 km/h [60 mph], dummy variable for 105 km/h [65 mph], dummy variable for 113 km/h [70 mph]); and • A dummy variable for the year 1999. Results of the crash frequency modeling effort are shown in Table 2-4. As shown, the median width is negatively

15 Variable NB RENB NM Constant –1.551 (0.181)† –0.118 (0.391) –1.500 (0.251)† ADT less than 5,000 veh per lane daily, indicator –1.398 (0.186)† –1.373 (0.190)† –1.381 (0.312)† ADT between 5,000 and 10,000 veh per lane daily, indicator –0.233 (0.158) –0.266 (0.157)‡ –0.298 (0.290) Median width between 30 and 40 ft, indicator 0.463 (0.206)† 0.368 (0.215)‡ 0.432 (0.309) Number of horizontal curves per kilometer –0.309 (0.128)† –0.325 (0.141)† –0.502 (0.262)‡ Length of section (km) if median width is less than 40 ft, 0 otherwise 0.281 (0.047)† 0.278 (0.062)† 0.175 (0.052)† Length of section (km) if median width is between 40 and 60 ft, 0 otherwise 0.526 (0.065)† 0.502 (0.070)† 0.292 (0.068)† Length of section (km) if median width is greater than 60 ft, 0 otherwise –0.358 (0.060)† –0.343 (0.065)† 0.105 (0.026)† Difference between maximum and minimum shoulder width is greater than 4 ft and the number of horizontal curves is greater than two per section, indicator 0.542 (0.321)‡ 0.489 (0.285)‡ 0.486 (0.580) Roadway friction factor if number of horizontal curves is greater than 1.08 per mi, 0 otherwise 0.011 (0.004)† 0.010 (0.005)† 0.009 (0.006) Washington State Route 2, indicator –2.093 (1.098)‡ –1.973 (1.371) 0.271 (0.587) Washington State Route 16, indicator –1.338 (0.581)† –1.290 (0.792) –1.188 (0.746) Washington State Route 90, indicator –0.722 (0.199)† –0.732 (0.195)† –0.560 (0.341) Washington State Route 205, indicator –1.814 (1.055)‡ –1.756 (1.150) –8.815 (0.533)† α 0.447 (0.172)† 0.258 (1.074) a 128.780 (312.380) b 34.514 (90.241) In L(=0, α=1), naïve model ** ** –827.556 In L at NB values — — –883.746 In L at convergence –711.931 –715.801 –613.078 Notes: Standard errors are given in parentheses. An “indicator” variable is 1 or a specified quantity if the condition holds and 0 otherwise. The NB and RENB model results presented elsewhere (6) are presented here for comparison with the NM model results. † = Significance at the 95% level by the two-tailed s-test: ‡ = significance at the 90% level by the two-tailed test. a,b = parameters of the beta distribution used in the RENB model: ** = information not available. Table 2-3. NM model coefficient estimation results for median crossover accident frequency (15). correlated with crash frequency in all models. This indicates that as the median width increases, the crash frequency decreases. Ordered multinomial logistic regression models were used to develop crash severity models for all four crash types described previously. The variables considered in these mod- els included the following: • Five levels of crash severity (K: fatal injury, A: incapacitating injury, B: nonincapacitating injury, C: possible injury, O: property damage only); • Dummy variable for year 1999; • Median width (ft); • Logarithm of ADT; • Number of lanes; and • Posted speed limit (dummy variable for 96 km/h [60 mph], dummy variable for 105 km/h [65 mph], dummy variable for 113 km/h (70 mph]). None of the explanatory variables used in the crash sever- ity models were found to be statistically significant, there- fore, the observed crash severity distributions were used in

16 the economic analysis. The severity distributions for each of the four crash types are shown in Table 2-5. The crash frequency models and severity data were used to estimate B/C ratios for both concrete and high-tension cable median barrier in Texas. Figure 2-9 shows a potential guideline for concrete median barrier based on B/C ratios. As shown, the B/C ratios increase from lower left to upper right. Zone No. 4 includes divided, limited-access roadways with low traf- fic volumes and the entire range of median widths considered in the study. The B/C ratios in Zone No. 4 were less than 2.0, thus the combination of traffic volume and median width was considered a lower priority for longitudinal barrier consider- ation than the other zones. Zone No. 1 includes average annual daily traffic volumes between 70,000 and 125,000 vehicles per day and median widths between 0 and 60 ft. In Zone No. 1, various median width–traffic volume combinations Covariate (coefficient) Crash frequency model No barrier With barrier Cross-median crashes Other median- related crashes All median- related crashes Hit-median- barrier crashes Offset = exposure (in MVMT) = v1 (= 365 * AADT * Segment Length / 1,000,000) —* — — — Intercept term Overall intercept (β0) Dummy variable for 1999: 1 if 1999 and 0 if 1998 (β1) –3.779 (±0.48) 1.163 (±0.14) –2.239 (±0.07) –0.068 (±0.05) –1.771 (±0.07) –0.031 (±0.001) –1.740 (±0.99) –0.018 (±0.06) Median width (in ft) (β2) –0.011 (±0.003) –0.002 (±0.001) –0.006 (±0.001) –0.013 (±0.002) Log (AADT) (β3) (AADT in 1,000s) — — — — Number of lanes (= β4) –0.293 (±0.09) — — — Posted speed limit (mph) Dummy variable for 60 mph (= 1 if 60 mph; = 0 if otherwise) (β5) Dummy variable for 65 mph (= 1 if 65 mph; = 0 if otherwise) (β6) Dummy variable for 70 mph (= 1 if 70 mph; = 0 if otherwise) (β7) –0.139 (±0.54) 0.500 (±0.16) 0.284 (±0.18) –0.342 (±0.17) –0.126 (±0.06) –0.079 (±0.07) –0.575 (±0.08) –0.075 (±0.07) –0.007 (±0.07) –0.063 (±0.10) –0.188 (±0.09) 0.004 (±0.09) Inverse dispersion parameter Inverse dispersion parameter for this model (Ψ) Inverse dispersion parameter for worst possible model of crash frequency (Ψ0freq) 0.727 (±0.17) 0.158 (±0.02) 1.388 (±0.12) 0.429 (±0.02) 1.956 (±0.16) (0.466 (±0.02) 1.464 (±0.13) 0.367 (±0.02) Goodness-of-fit measures Deviance information criterion/sample size (DIC/n) 0.39 0.78 1.71 0.69 2.54 0.76 2.14 0.75 Notes: All models were structured using the full Bayes framework with noninformative priors (or hyperpriors). Parameters (β and Ψ) were estimated by using Markov Chain Monte Carlo techniques, and the values shown in the table are their posterior means. Values in parentheses are the estimated one standard error of parameters to their left based on the posterior density of the parameter. —* indicates not statistically significant at a 10% significance level. )/)(/(R freqfreq 01 111 Table 2-4. Posterior mean and standard error of estimated parameters of Texas median safety crash frequency models (16). Barrier and crash type N Number and percentage of crashes by severity type K % A % B % C % PDO % No Median Barrier Cross-median 346 73 21.1 73 21.1 82 23.7 58 16.8 60 17.3 Other Median-related 3,046 71 2.3 272 8.9 639 20.9 734 23.9 1,348 44.0 With Median Barrier All Median-related 3,672 36 1.0 190 5.2 681 18.5 1,098 29.9 1,667 45.4 Hit-Median-Barrier 2,714 13 0.5 128 4.7 490 18.0 835 30.8 1,248 46.0 N = total number of crashes K = fatal A = Incapacitating injury B = Nonincapacitating injury C = Possible injury PDO = Property damage only Table 2-5. Texas median crash severity distribution (16).

17 produced B/C ratios greater than 10.0. As such, divided high- ways in Zone No. 1 without longitudinal median barrier were considered the highest priority for median barrier installa- tion. Further, it was recommended that road sections with a mean B/C ratio greater than 10 be given the highest priority when installing concrete median barriers. To develop a potential guideline for the installation of high-tension cable barriers, a favorability ratio was developed. A favorability ratio was defined as the ratio of the high-tension cable barrier’s mean B/C ratio over the concrete barrier’s mean B/C ratio. Table 2-6 shows the calculated favorability ratios for various median widths and traffic volumes. A favorability ratio of 1 indicated that concrete and high-tension cable bar- riers had the same mean B/C ratio and higher ratios suggested increased favorability of using the high-tension cable barrier over the concrete barrier in terms of the mean B/C ratios. Figure 2-9. Benefit-cost ratios based on Texas study (16). Table 2-6. Favorability ratios from Texas study (16).

18 Miaou et al. (16) recommended considering high-tension cable barriers only when the favorability ratio exceeded 2. Noyce and McKendry (17) investigated the magnitude of and factors affecting median crossover crashes in Wisconsin using data from freeways and expressways. In 3 years (2001 through 2003), there were 631 median crossover crashes on four Inter- states and 17 other freeways and expressways in Wisconsin. Of these, 81 percent (511 of 631) were single-vehicle crashes. In such instances, single-vehicle crashes involve motorists run- ning off the road to the left and entering the median; however, a collision with a vehicle traveling in the opposing travel lanes did not result. The crossover crash severity distribution was as follows: • 6.5 percent fatal (41 of 631); • 53.2 percent injury (336 of 631); and • 40.3 percent property damage only (254 of 631). The most common initial cause of median crossover crashes was lost control due to weather (44.0 percent), lost control on dry pavement (41.7 percent), and vehicle collision (11.1 percent). 2.3 Other State Highway Agency Median Safety Research 2.3.1 North Carolina Population growth in North Carolina has spawned an increase in the number of vehicle-miles traveled. This increase in travel is also associated with an increase in cross-median crashes on the Interstate and freeway system. The Across Median Safety Study (18) identified and investigated over 800 cross-median crashes along nearly 2,212 km (1,375 mi) of Interstate and non Interstate freeway facilities in North Carolina from January 1, 1994 through June 30, 1997. The study showed that although cross-median crashes make-up less than 5 percent of the injuries on the entire Interstate system, these crashes comprise nearly 23 percent of all fatal injuries and 13 percent of all severe injuries (18). Only 27 per- cent of all cross-median crashes on North Carolina freeways occurred where a barrier is warranted according to AASHTO criteria; 58 percent occur where barrier is optional; 15 per- cent occur where barrier is not normally considered. The cross-median crash data are shown in Figure 2-10 with the AASHTO warrants indicated. In 1998, the Traffic Engineering and Safety Systems Branch initiated a three-pronged proactive approach to prevent cross- median crashes in North Carolina. The result of the first phase of the plan showed 23 high-priority locations along 386 km (240 mi) of freeway where cross-median crashes represented an unusually high concentration of accidents. It was recom- mended that some type of positive barrier protection be installed immediately in these locations to prevent further accidents. The second phase of the plan consisted of priori- tizing and systematically protecting all freeway sections with median widths less than 21 m (70 ft). A hazard index that linked ADT, speed limit, and median widths was developed to help create a priority ranking system. In all, over 100 addi- tional sections were identified as potential protection loca- tions. The final phase of the plan consisted of revising the Figure 2-10. Cross-median crashes on North Carolina divided freeways (18).

19 state’s median design policy so that no more freeways could be built with median widths less than 21 m (70 ft) (18). Of the 23 locations identified in the first phase of the plan, the total estimated cost of installing some type of positive barrier was nearly $16 million; the 100 locations subsequently identified for protection in the second phase of the plan could cost an additional $65 million based on an estimated unit cost of $49,720 per km ($80,000 per mi) (18). 2.3.2 California Beginning in 1947, the California Department of Transpor- tation (Caltrans) has been constantly reviewing median instal- lations and the effect that they have on accident frequency and severity. A major study performed in 1958 related traffic volumes to median widths, thus establishing a barrier war- rant policy. This 1958 study called for barrier consideration on roadways carrying volumes in excess of 60,000 vehicles per day and having median widths less than 11 m (36 ft) (19). Cable barriers were considered positive protection for median widths between 5 and 11 m (16 and 36 ft) while metal beam barriers were used in medians less than 5 m (16 ft) in width. Subsequent evaluations took place, which only confirmed that the barriers were successful in reducing fatal cross-median crashes. In 1968, the California Department of Transportation used a diminishing-return analysis and concluded that the placement of barrier would be concentrated at locations with medians up to and including 14 m (45 ft) in width (19). A diminishing-return analysis estimates the greatest return in reduced median-related crashes for the barrier investment cost. In California, the total cross-median fatal-and-injury crashes eliminated were compared to the miles of median barrier required on high-speed, divided highways. Based on comparing the cumulative crashes to cumulative miles of barrier required to prevent such crashes, a point of diminish- ing return is reached where barrier installation costs would outweigh the cost in crash cost benefits. The median width/ volume criteria developed by Caltrans were later adopted by AASHTO with some modifications. This policy has gone rel- atively unchanged in California since the late 1960s. In 1997, Caltrans again conducted a study to investigate the benefits of their median barrier warrant criteria. The study evaluated the traffic volume/median width warrant as well as an accident study warrant of 0.3 cross-median accidents of any severity per km per year (0.50 cross-median accidents of any severity per mile per year) or 0.07 fatal cross-median accidents per km per year (0.12 fatal cross-median accidents per mile per year) (19). The volume/median width warrant was evaluated based on a diminishing-return analysis, as well as a benefit-cost analysis that accounted for the increased number of accidents that occurred when installing barrier to decrease the severity of an accident. This particular study analyzed data over a 5-year period beginning in 1991. The benefit-cost analysis used in the study was based on a human capital method where fatal accidents were valued at $850,000 per accident, injury accidents were valued at $17,200 per accident, and property-damage-only accidents were valued at $3,700 per accident. In addition, the cost of installing median barrier on California freeways was valued at $270,000 per mile. To complete the benefit/cost analysis, the severity of hit-barrier accidents versus cross- median accidents was determined. The data collected for the study contained sites where barrier was present (after condition) and where barrier was not present (before con- dition). Many combinations of median width and average daily traffic were studied, and the results are shown in Fig- ure 2-11. Ultimately, the benefit/cost ratio determined in relation to extending the volume/width warrant from 14 m (45 ft) up to 23 m (75 ft) was 1.10. In all, this modified war- rant required 628 km (390 mi) of newly installed barrier estimated to provide a reduction of 15 fatal accidents per year, an increase of 320 injury accidents per year, and an increase of 550 property-damage-only accidents per year. The diminishing-return analysis served to verify that a com- bination of median width and average daily traffic produces the best results for the study type. 2.3.3 Florida Turnpike In February 1999, the Florida Turnpike Traffic Operations Center performed a study to investigate cross-median colli- sions along the Florida Turnpike (SR 91) and the Homestead Extension of the Florida Turnpike (SR 821). The study evalu- ated fatal cross-median collisions along 502 km (312 mi) of the turnpike for the years 1995 to 1997. As a result of the study, the Turnpike Authority contracted HNTB in March 1999 to perform a comprehensive study so that a median safety improvement program could be created. The data collection effort consisted of identifying and reviewing crash reports, interviewing individuals from various organizations, conducting detailed field reviews, and retriev- ing physical, geometric, and operational characteristics of the Florida Turnpike (20). An interview questionnaire was sent to 22 individuals repre- senting traffic engineering organizations in Florida. From the responses received, the following statements best summarize the findings (20): • Standing water on the turnpike shoulders could result in an increased number of cross-median collisions or injury- type accidents as a result of impacting a barrier wall. • Illegal U-turns are a concern on the Florida Turnpike as motorists seek to avoid the toll.

20 • The Florida Highway Patrol should have a line item on their accident report forms that clearly indicates a cross- median crash. • The benefit of using shoulder rumble strips along shoul- ders adjacent to median barrier walls should be evaluated. • Concrete median barrier should be used to protect narrow medians with paved or hard-surfaced shoulders, and guard- rail should be used when there is a wider grass median. A typical cross section of the Florida Turnpike contains two basic median types as well as two basic lane configurations that consist of either four or six through lanes. The depressed median configuration consists of sections that are 12, 16, 20, and 27 m (40, 52, 64, and 88 ft) in width. The continuous median barrier section is typically 6 m (20 ft) wide (20). In both the depressed and barrier median sections of the turn- pike, the frequency of crashes steadily rose during the study period. The number of cross-median collisions had risen from 17 in 1995 to 93 in 1998. Similarly, the number of median bar- rier crashes increased significantly from 151 in 1995 to 380 in 1998. In the case of both the crossover and barrier crashes, both the fatality and injury rates increased. To identify the highest crash locations, the turnpike was divided into 1-mile incre- ments, and the number of crashes, cross-median collisions, cross-median fatal collisions, and median barrier crashes were plotted and analyzed for each 1-mile segment (20). From the analysis, five priority locations were pinpointed as high crash locations where median barrier could be expected to reduce median-related fatal accidents by nearly 75 percent. The final phase of the study included a benefit/cost analysis to determine the most economical implementation program for improving median safety along the Florida Turnpike. In all, 10 different combinations were considered and ana- lyzed. They considered barrier placement locations, barrier type, and median width and cross-slopes. The benefit/cost ratios ranged from 3.3 to 37.5 for various treatment com- binations. The breakeven analysis showed that it would take 15 to 25 years, depending on the treatment type, to begin realizing a benefit from a median safety improvement pro- gram. Finally, 55 to 107 fatal crashes could be prevented when reaching the cost breakeven point (20). In addition to the Florida Turnpike Commission, the Florida Department of Transportation evaluated median-related crashes in the late 1990s. In 1991, the Florida Department of Transportation adopted the policy of installing longitudinal median barrier on all divided Florida highways if the median width were less than 20 m (64 ft). An examination of cross- median crashes based on 5 years of crash data (1995 through 1999) was undertaken to determine the typical characteristics of these crashes and to recommend methods to reduce their Figure 2-11. California freeway median barrier warrant (19).

21 frequency and severity. During the data collection period, it was estimated that between 300 and 750 cross-median colli- sions occurred on Florida highways. The following charac- teristics of cross-median crashes were identified by review of hardcopy police accident reports (21): • Approximately 19 percent involved, or were suspected to involve, alcohol. • About 2 percent of crashes involved a truck as the crossing vehicle. • Nearly 78 percent of crashes occurred when the crossing vehicle’s speed was within 5 mph of the posted speed limit. • Prevailing weather conditions were good in 75 percent of crashes—83 percent of these crashes were the result of driver error and avoidance maneuvers. • About half of the crashes that occurred during adverse weather conditions involved hydroplaning and the other half were the result of driver error and avoidance maneuvers. • Approximately 62 percent of all cross-median crashes occurred within one-half mile of interchange ramp termini, and approximately 82 percent occurred within 1 mile of ramp termini. A cost-effectiveness analysis revealed that median barri- ers should reduce the fatality rate and societal costs due to cross-median crashes by about 50 percent; however, the over- all crash frequency and injury rates will increase by 600 and 28 percent, respectively. When installed in areas without a crash history, the barrier may not offer any cost benefit over the no-barrier alternative. It was recommended that the 20-m (64-ft) median barrier warrant be retained and the barrier is evaluated based on crash history. Also, crash locations within 1 mile of ramp termini were investigated and locations with a crash history are being considered for barrier installation. 2.3.4 Georgia In the summer of 2000, a panel of experts on median design and safety at the Georgia Department of Transporta- tion met to revise the department’s median guidelines. They concluded that several factors would be used to address the applicability of median treatments. These factors are classifica- tion of roadway, number of lanes, base year traffic, design year traffic, posted speed limit, design speed, and accident/crash data. Highways were grouped into functional classifications. Georgia median guidelines for each classification considered are stated in the following subsections (22). 2.3.4.1 Urban Interstates The panel determined that all urban Interstates will have positive barrier separation. In addition, all urban multilane roadways that interchange with an Interstate will have a raised median for a distance of 300 m (1,000 ft) from the ramp termini or to the first major intersection. 2.3.4.2 Rural Interstates For rural Interstates, the panel concluded that all would require a depressed median as specified in the AASHTO Green Book. In areas where right-of-way restrictions exist, the guide- lines suggest positive barrier separation to be incorporated. In addition, all rural multilane roadways interchanging with the Interstate will have a raised median for a 300-m (1,000-ft) dis- tance from the ramp termini or to the first major intersection. 2.3.4.3 Arterials The panel derived several guidelines to be incorporated for median treatment of arterial highways. These median policies are as follows: • All arterial highways with design speeds or posted speed limits less than or equal to 72 km/h (45 mph), and having base year traffic volumes less than 18,000 vehicles per day, and design year traffic volumes equal to 24,000 vehicles per day will require a five-lane cross-section that includes a flush median. For new alignments, an additional 6 m (20 ft) of right-of-way will be purchased to incorporate a future 6-m (20-ft) raised median. The need for implementing a 6-m (20-ft) raised median will be determined by monitor- ing accidents and traffic volumes over a 5-year cycle. • A 6-m (20-ft) raised median will be constructed on all urban arterials with base year traffic volumes greater than or equal to 18,000 and design year traffic volumes greater than or equal to 24,000 with a design speed less than or equal to 72 km/h (45 mph). • All arterial highways with posted speed limits greater than or equal to 88 km/h (55 mph) or design speeds greater than or equal to 80 km/h (50 mph) will require the incorpo- ration of a 13-m (44-ft) depressed median. If this is not feasible, a positive barrier system must be implemented. • All multilane facilities with three or more lanes in one direc- tion of travel must include positive separation of opposing traffic using a median. The type of median to implement shall be determined from the guidelines stated above. 2.3.5 Washington The purpose of a Washington State Department of Trans- portation study (23) was to evaluate the frequency and severity of cross-median crashes on divided highways. A benefit-cost analysis was used to develop revised median barrier instal- lation guidelines and to rank or prioritize median barrier

22 improvement projects. In all, 1,089 km (677 mi) of Wash- ington State highways were studied. Each section examined was a multilane, divided highway with full control of access and with a depressed or unprotected median. Additionally, posted speed limits were greater than 72 km/h (45 mph) and average daily traffic volumes were greater than 5,000 vehicles per day. Five (5) years of crash data (1996 through 2000) were examined and a total of 642 cross-median crashes identified. Prior to the research, the AASHTO median barrier warrant criteria were used to evaluate the need for median barrier. Crash analyses showed the following crash frequencies (23): • At highway locations with 0- to 9-m (0- to 30-ft) medians, three cross-median crashes in 5 years; • At locations with 9- to 12-m (31- to 40-ft) medians, 273 crashes; • At locations with 12- to 15-m (41- to 50-ft) medians, 100 crashes; • At locations with 15- to 18-m (51- to 60-ft) medians, 9 crashes; • At locations with 19- to 21-m (61- to 70-ft) medians, 16 crashes; • At locations with 22- to 24-m (71- to 80-ft) medians, 153 crashes; and • At sites with medians wider than 24 m (80 ft), 88 crashes. Based on the analysis, there was a clear indication that the installation of median barrier offered benefits that exceeded costs for medians up to 15 m (50 ft) in width, regardless of the barrier type being installed. Currently, median barrier is recommended on all access-controlled, multilane highways with posted speeds greater than 45 mph if the median is less than 15 m (50 ft) wide. The barrier type is determined on a project basis and medians with lower posted speeds or with widths greater than 15 m (50 ft) are considered as candidate median barrier locations based on crash histories. Shankar et al. (24) validated the results using count regression and societal cost modeling methods. 2.4 Median Barrier Effectiveness Evaluations As previously discussed, there are nine general median barrier types commonly recognized for installation on access- controlled, high-speed, divided highways. The concrete median barrier has several variations, including the New Jersey shape, F-shape, and single-slope (or vertical wall). Also, other forms of high-tension cable barrier (e.g., Brifen Wire Rope Safety Fence) are now being used by state transportation agencies. The results of a survey of state transportation agency median barrier type use are shown in Figure 2-12 (25), which shows the number of responding agencies that approve the various median barrier types on high-speed, divided highways. Published research related to the median barrier types used by various agencies is described in the remainder of this section. Figure 2-12. Approved median barrier use by state transportation agencies (25). 28 25 24 15 14 12 6 4 3 2 0 5 10 15 20 25 30 Strong-post W-beam New Jersey Concrete Barrier F-shape Concrete Barrier Strong-post Thrie Beam Single Slope Concrete Barrier Three-strand Cable Modified Thrie Beam Brifen Wire Rope Box Beam Weak-post W- beam Median Barrier Type N um be r o f S TA s U sin g Ba rr ie r

23 2.4.1 Cable Barrier In December 1996, the Oregon Department of Trans- portation installed two sections of cable barrier along I-5 between Salem and Portland to reduce cross-median col- lisions. Sposito and Johnston (26) evaluated the effects of cable median barrier on I-5 in Oregon using historical crash data. The total length of two separate evaluation sections was 14.5 km (9.0 mi). The average median width was 15 m (50 ft). The posted speed limit was 105 km/h (65 mph) and the aver- age daily traffic (ADT) for 1997 varied from 71,900 to 74,700 vehicles per day on the two sections. A simple before-after crash analysis was performed. The data before the installation of median barriers was from 1987 to 1996 while the after- period data was from December 1996 through March 1998. By comparing the crash rates before and after the installa- tion of barrier, it was concluded that the fatality rate dropped (from 0.6 per year to 0) but the injury crash rate increased (from 0.7 per year to 3.8 per year). Also, by investigating the barrier impact accidents from December 1996 to March 1998 using maintenance records and police crash reports, it was concluded that the cable median barrier system was effec- tive in preventing crossover accidents at the researched sites because 21 potential crossovers (40 percent of the total bar- rier impacts) were prevented by the barriers. The annual cost of a cable median barrier system would be less than that of a concrete barrier system. Monsere et al. (27) performed a subsequent evaluation of median-related crashes before and after installation of cable median barrier on freeway facilities in Oregon. The evalua- tion section was 35.2 km (21.9 mi) long on I-5. The average median width was 15 m (50 ft) and the average width of the inside paved shoulder was 3 m (10 ft). The posted speed limit was 105 km/h (65 mph) and the ADT for the analysis period varied from 66,000 to 82,600 vehicles per day. The data before the installation of median barriers were from December 1993 to December 1996 and the data after the installation of median barriers were from May 1998 through May 2001. Target crash types included: (1) median crossover crash; (2) striking bar- rier crash, and (3) crashes unrelated to barrier. Types (1) and (2) were considered in the analysis and were summarized by severity using the KABCO scale. By comparing the number of crashes for each severity level before and after installation of the cable barrier system, it was concluded that the cable median barrier was effective in preventing median cross- over crashes. It was estimated that 105 potential crossovers (45 percent of the total barrier impacts) were prevented by the longitudinal cable median barrier. Hunter et al. (28) studied crash rates and crash types for three-strand cable median barrier installed on I-40 in North Carolina. Crash data used to develop the model contained crash counts along with associated roadway characteristics from 1990 to 1997 involving 6,111 crashes. Three-strand cable median barrier was not installed on high hazard sections until after 1994. Therefore, researchers were able to compare the before and after effects of the cable barrier installment. Three-strand cable barrier was installed on a 13.7-km (8.5-mi) section of I-40 between Raleigh and Durham. The median width along this segment of highway ranged from 13 to 20 m (44 to 64 ft). Approximately 80 percent of the cable was installed as the double-run type except in the eastern sec- tion where the median width was 20 m (64 ft). In the place- ment of the single-run cable at the median center, the North Carolina Department of Transportation (NCDOT) recom- mends that it should not be used on narrow medians or on medians with slopes greater than 1V:6H. In developing the model, two populations were identified. One population consisted of sections treated with guiderail, while the other population consisted of the entire North Carolina Interstate system not treated with cable barrier, known as the reference population. Analyses were conducted for several different crash types. For total crashes, the reference group had a higher expected crash per mile rate than the pre-treatment group even though the reference group had lower crashes per mile than the treat- ment sites. In addition, a significant increase in total crashes was realized from pre-treatment years to post-treatment years, but only at a level equivalent to the rest of the Inter- state system (28). For serious injury and fatal accidents, the analysis showed that these types of crashes started to decrease during the transition year (1994), and continued in the post-treatment period. The analysis also revealed lower post-treatment rates when compared to sections in the reference population. Run-off-road-left-hit-fixed-object accident models revealed that there was a significant increase in 1994 (transition year), which continued through the post-treatment period. This confirmed expectations because installing cable barrier into the median reduces the effective clear recovery width. These types of crashes stayed relatively the same between the refer- ence population and the pre-treatment period. Rear-end crashes were revealed to significantly increase from the pre-treatment years to the post-treatment years. The mod- eling also exhibited that there were not significant changes in ran-off-road-left overturn crashes from the transition period to the post-treatment period. A severity index was calculated for crash types during each of the study years. NCDOT developed the mathematical for- mula used to calculate the severity index for this study (28). Severity indexes can fall into one of five categories: low, aver- age, moderate, high, and very high. Severity index values for the pre-treatment years fall into the moderate severity category. For the transition year and post-treatment years, the severity index fell into the average severity index category. In other

24 words, this states that the severity index of a crash decreased after cable barrier was installed in the median. Davis and Pei (29) reconstructed two cross-median crashes using Markov Chain Monte Carlo (MCMC) and Bayesian methods to verify that simulation could be used to produce estimates of impact severity if cable barrier had been in use at the time of the crash event. The impact severity estimates were computed for a barrier located at the edge of the shoul- der and at the center of the median. Once the crashes were verified using the reconstruction method, six cross-median and three rollover crashes were considered in the impact severity analysis. The median widths for the nine cases ranged from 13.1 to 21.8 m (43.0 to 71.4 ft). The posted speed limit ranged from 80 to 113 km/h (50 to 70 mph) and the ADT ranged from 8,900 to 42,000 vehicles per day. In all cases, the AASHTO Roadside Design Guide (2) would consider median barrier optional or not normally considered. The results indi- cated that had a barrier been present, the impact severity of the crossing vehicle would have been below the maximum set forth in NCHRP Report 350 (30). Bergh et al. (31) described the Swedish National Road Administration’s program to improve traffic safety on exist- ing 13-m (42-ft) wide two-lane roads. The program con- sisted of converting roadways with two travel lanes (3.75 m or 12.3 ft each) and two paved shoulders (2.75 m or 9.0 ft each) to a three-lane roadway with 3.75-m (12.3-ft) wide out- side travel lanes and a 3.5-m (11.5-ft) wide middle lane with 1.0-m (3.1-ft) paved shoulders. Traffic flows changed direc- tion every 1.0 to 2.5 km (0.6 to 1.6 mi) in the middle lane and the opposing travel lanes were separated using a cable median barrier (known as 2+1 roads with a cable barrier). Speed performance, traffic safety, driver attitudes, and main- tenance issues were all included in the evaluation. The find- ings were as follows: • The average travel speed increased by 2 km/h (1.2 mph) after converting to a 2+1 road with cable median barrier when the posted speed was 90 km/h (55 mph). • On roads with a posted speed limit of 110 km/h (68 mph), the average travel speed on the two-lane sections was 111 km/h (69 mph) and the average travel speed in the single lane sections was 106 km/h (66 mph). • Capacity was estimated to be 1,500 to 1,550 vehicles per hour per lane after the conversion; the capacity for the pre- vious cross-section was approximately 1,800 vehicles per hour per lane. • The reduction in severe injury crashes was estimated to be 40 to 55 percent. • The reduction in fatal crashes was estimated to be 65 to 70 percent. • The median cable barrier crash rate was 0.6 crashes per mil- lion axle-pair km (0.97 crashes per million axle-pair mi). • After 2 years, drivers began to prefer the 2+1 cable median barrier design over other road conversion types (e.g., 2+1 with pavement markings). • Maintenance costs increased after installation of the cable median barrier. 2.4.2 Concrete Safety Shape Barriers McNally and Yaksich (32) showed that New Jersey barrier installation decreased the frequency of fatal accidents by 31 per- cent while the frequency of nonfatal and non-injury accidents increased by 9.2 and 2.4 percent, respectively. Elvik (33) summarized 32 previous studies using meta- analysis to evaluate the effects of median barriers, guardrails, and crash cushions on crash rate and crash severity. The results showed that median barriers increased crash rate but reduced crash severity, while guardrails reduced both the crash rate and crash severity. The effects of crash cushions were not conclusive. The best estimate of the safety effects of median barriers was a 30 percent increase in crash rate, a 20 percent reduction in fatalities given a crash has occurred, and a 10 percent reduction in nonfatal injuries given a crash has occurred. The safety effects of guardrails were a 45 per- cent reduction in fatalities and a 50 percent reduction in non- fatal injuries, given a crash has occurred.