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10 A 1977 Purdue University study by Van Maren (13) devel- MEDIAN OPENING LENGTH oped relationships between geometric and traffic volume vari- ables and accident experience at divided highway intersec- The only literature found on the relationship of median tions. Van Maren found no statistically significant relationship opening length to safety is the research by Harwood et al. in between median width and intersection accident rate. The NCHRP Report 375: Median Intersection Design (7). That author speculated that this finding may have resulted because report addressed the effect of median opening length on unde- of the limited range of median widths (9 to 18 m or 30 to 60 ft) sirable driving behavior. Most undesirable driving behavior at that were evaluated. However, this range includes most of the divided highway intersections arises from the competition for rural divided nonfreeways that have been built by highway limited space on the median roadway between drivers travel- agencies since the 1950s, including current practices. ing through the median in the same direction. NCHRP Report Research sponsored by the Michigan Department of Trans- 375 found that the frequency of undesirable driving behavior portation involved the collection and analysis of data for increases as median opening length increases at rural inter- 1,503 km (934 mi) of Michigan state highways (14). Acci- sections and decreases as median opening length increases at dents on divided highway segments were compared with suburban intersections. highway segments (mostly five-lane) with two-way left-turn lanes (TWLTLs). The analysis of the accident data with SAFETY EFFECTS OF MEDIAN TREATMENTS respect to the width of the median did not show any mean- (RAISED/DEPRESSED/FLUSH/TWLTL) ingful differences for divided highway segments that did not have traffic signals. The research found that divided high- The treatment of roadway medians influences the safety ways with traffic signals may have lower accident rates with and operational experience of a roadway as well as the access wider medians. However, the data were insufficient for con- provided to adjacent developments. The four major types of clusive findings on this issue. median treatments are as follows: The Florida Median Handbook (8) suggests that the appro- priate median width is a function of the purpose which the Raised median--A raised median is a nontraversable median is to serve in a particular application, such as the median separated from the traveled way by curbs. Raised following: medians are used where it is desirable to separate traffic traveling in opposite directions and limit left-turn move- Separation of opposing traffic streams, ments. The area within the median can be either con- Pedestrian refuge, crete or turf. Left turn to side street, Depressed median--A depressed median is a nontra- Left turn out of side street, versable turf median that separates traffic in opposite Crossing vehicles, directions of travel and limits left-turn movements. As U-turns, and the name implies, a depressed median usually slopes Aesthetics and maintenance. away from the roadway to provide proper drainage. A depressed median has no curbs; the median is typically Table 1 presents a summary of standards and recommenda- separated from the traveled way by pavement markings tions for median widths, as presented in the Florida Median and shoulders. Handbook. The handbook recommends that extremely wide Flush median--A flush median is a paved area, at the medians are needed to accommodate U-turn maneuvers by same grade as the traveled way, that may be marked as a all design vehicles other than passenger cars. median or as a center two-way left-turn lane (see below). TABLE 1 Minimum and recommended median widths (8) Median width Roadway type Speed m (ft) Reconstruction Project 40 mph or less 5.0 (15.5) Minimum Reconstruction Project 45 mph 6.0 (19.5) Minimum Reconstruction Project 50 mph 7.0 (22.0) Minimum Four-lane highways with medians All 9.0 (30.0)--single left Recommended expecting significant U-turns and turns directional median openings with 12.6 (42.0)--dual left excellent positive guidance turns Six-lane highways with medians All 7.0 (22.0)--single left Recommended expecting significant U-turns and turns directional median openings with 10.6 (34.0)--dual left excellent positive guidance turns

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11 Two-way left-turn lane (TWLTL)--A TWLTL is a cen- and-after studies of the same roadway and some comparing ter lane used for left turns from both directions of travel. accident rates for the two basic types of roads. The accident At intersections, there is often a transition to conven- rate comparisons from the various studies are summarized in tional left-turn treatments. Table 4. In 15 out of the 16 comparisons shown in Table 4, the accident rates were reduced when a nontraversable median was The literature on these median treatments is extensive. Sev- installed in place of a TWLTL. NCHRP Report 420 concluded eral NCHRP reports and other sources present safety compar- that nontraversable medians appear safer than TWLTLs. isons of alternative median treatments. NCHRP Report 282: Multilane Design Alternatives for Research by Bonneson and McCoy in NCHRP Report Improving Suburban Highways (22) presents a comparison 395: Capacity and Operational Effects of Midblock Left- of the safety, operational, and cost characteristics of selected Turn Lanes (15) considered the relative traffic operational multilane design alternatives for use in suburban areas. Advan- and safety performance of cross-section for arterials and tages and disadvantages of each alternative are provided to highways that are undivided, divided by a median, or divided assist in the selection of the most appropriate design for a by a center TWLTL. Table 2 presents a comparison of these given condition. The report states that the four-lane divided three alternative cross-sections, indicating which cross-section design alternative is best suited for use on major arterials is preferred with respect to operational, safety, access, and with high volumes of through traffic and less than 45 drive- other factors. NCHRP Report 395 reviewed the relative ways per mile. The five-lane TWLTL design alternative is safety performance of arterials with different cross-sections. most appropriate for suburban highways with commercial Table 3 summarizes the safety performance of these cross- development, driveway densities greater than 45 driveways sections as reported by the following key sources in the liter- per mile, low-to-moderate volumes of through traffic, high ature: Bowman and Vecellio (16), Chatterjee et al. (17), Parker left-turn volumes, and/or high rates of rear-end and angle (18), Squires and Parsonson (19), McCoy and Ballard (20), accidents associated with left-turn maneuvers. Thus, NCHRP Walton and Machemehl (21), and NCHRP Report 282 (22). Report 282 does not make a blanket statement about the rel- NCHRP Report 420: Impacts of Access Management Tech- ative safety of nontraversable medians and TWLTLs, but niques (4) presents a summary of individual studies that have indicates that each has appropriate applications. analyzed the safety benefits of replacing TWLTLs with non- NCHRP Report 330: Effective Utilization of Street Width traversable medians on undivided highways. Eleven studies on Urban Arterials (23) evaluated various alternative strate- were reviewed: some where the benefits were based on before- gies for reallocating the usage of street width without chang- TABLE 2 Comparison of effects of three alternative cross-sections with differing midblock left-turn treatment types (15)

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TABLE 3 Comparison of safety performance of alternative midblock cross-section as reported by studies in the literature (15)

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TABLE 4 Synthesis of safety experience comparing TWLTLs with nontraversable medians by percent difference (4)

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14 ing the total curb-to-curb width. Table 5 presents the advan- ans had lower injury accident rates than either the TWLTL tages and disadvantages of four-lane divided roadways and or undivided cross-sections. five-lane roadways with TWLTLs. Research sponsored by the Michigan Department of Trans- Research for the Florida Department of Transportation portation involved the collection and analysis of data for was performed for five roadway segments within the Central 1,503 km (934 mi) of Michigan state highways (14). Accidents Florida area that underwent median modifications (2). Sev- on divided highway segments were compared with highway eral of these segments also had other improvements, such as segments (mostly five-lane) with TWLTLs. The divided high- the addition of a through or auxiliary lane. The research way segments in Michigan generally have directional U-turn results showed that the introduction of medians can greatly median crossovers that are also used for the indirect movement reduce collision potential and injuries. These reductions were of left-turning traffic. Divided highway segments were found found to occur as a result of the decrease in the number of to have lower accident rates than TWLTLs for nearly every conflict points. The research noted that conflict points are type of accident. The total accident rate (for all accident types) numerous along roadways with a continuous TWLTL. Con- for the divided highway segments was approximately 50 per- flict reductions with an associated decrease in collision poten- cent of the total accident rate for highways with a TWLTL. tial may be achieved by either reducing the number of median Divided highways that exclusively have directional U-turn openings or adding a median. median crossovers were found to have approximately the The FHWA sponsored research to quantify the safety effect same accident rate as divided highways that have conven- of raised curb, TWLTL, and undivided cross-sections on tional (bidirectional) median crossovers for unsignalized sec- vehicles and pedestrians (16). A total of 32,894 vehicle and tions of highways. Signalized divided highways with direc- 1,012 pedestrian accidents were analyzed from 234.8 km tional crossovers were found to have about 50 percent of the (145.9 mi) of unlimited access arterials in three large metro- accident rate of similar facilities with conventional median politan areas. The research found that streets with raised openings. However, the size of the data sample for this issue medians in both central business districts (CBDs) and subur- does not support conclusive findings. ban areas had lower pedestrian accident rates than TWLTLs The comparison between raised medians and TWLTLs has and undivided arterials. In suburban areas, arterials with raised- been the focus of numerous other research studies. Accident curb medians were found to have significantly lower accident data assembled by Chatterjee et al. (17) and by Parker (18) rates than TWLTLs for rear-end, right-angle, and left-turn col- indicate that raised-curb median segments have lower accident lisions. Raised-curb medians also were found to have signif- rates than TWLTL segments. Walton and Machemehl (21) icantly lower accident rates than undivided cross-sections for developed accident prediction equations for roadway seg- right-angle collisions. The research results also indicated ments with TWLTLs. Not enough data were available to that, in both CBDs and suburban locations, raised-curb medi- develop comparable equations for segments with raised medi- TABLE 5 Advantages and disadvantages of four-lane divided roadways and five-lane roadways with TWLTLs alternatives (23) Design alternative Advantages Disadvantages Four-lane divided 1. Provides additional lanes to 1. Required street width may roadways increase capacity for not be available through traffic movement 2. Increased delay to left- 2. Reduces rear-end and turning vehicles angle accidents associated 3. Indirect routing required with left-turn maneuvers for large trucks 3. Provides physical 4. Lack of operational separation to reduce head- flexibility due to fixed on accidents median 4. Provides a median refuge area for pedestrians Five-lane roadways with 1. Provides additional lanes to 1. Required street width may TWLTLs increase capacity for not be available through traffic movement 2. No refuge area in median 2. Reduces delay to through for pedestrians vehicles caused by left- 3. May generate safety turning vehicles problems at closely 3. Reduces frequency of rear- spaced driveways and end and angle accidents intersections associated with left-turn maneuvers 4. Provides spatial separation between opposing lanes to reduce head-on accidents 5. Increases operational flexibility