National Academies Press: OpenBook

Recent Roadway Geometric Design Research for Improved Safety and Operations (2012)

Chapter: Chapter Three - Elements of Design

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Suggested Citation:"Chapter Three - Elements of Design." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
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Suggested Citation:"Chapter Three - Elements of Design." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
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Suggested Citation:"Chapter Three - Elements of Design." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
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Suggested Citation:"Chapter Three - Elements of Design." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
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Suggested Citation:"Chapter Three - Elements of Design." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
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Suggested Citation:"Chapter Three - Elements of Design." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
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Suggested Citation:"Chapter Three - Elements of Design." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
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Suggested Citation:"Chapter Three - Elements of Design." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
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13 Overview Research during the decade led to new recommendations for SSD, which were later analyzed for probability of hazard (POH). An updated look at passing sight distance (PSD) compared Green Book guidelines with MUTCD guidelines for passing zone markings. With the increasing availability of appropriate technological design aids, a new emphasis on 3-D modeling was promoted to consider the interactions between horizon- tal and vertical alignments and their effects on the driver. Additional methods for 2-D analyses were also investigated. Researchers also revisited truck performance on crest curves and headlight performance on sag curves to compare current traffic characteristics with existing guidelines. Estimated safety benefits of selected design elements are also discussed in this chapter, based on the material contained in the AASHTO Highway Safety Manual (HSM); this chapter does not contain a comprehensive reproduction of the HSM guidance, but does provide examples. Readers desiring to obtain full details on the safety effects of various design elements and treatments, including the appropriate methodology for the proper applica- tion of HSM guidance, should consult that document. Sight DiStance Stopping Sight Distance Fambro et al. (2000) developed a SSD model to update the values used in the then-current 1994 Green Book. A compari- son of the existing SSD model with those used by other coun- tries showed that AASHTO’s SSD values and vertical curve lengths were longer than those used in most other countries. The researchers conducted field studies involving more than 50 drivers, 3,000 braking maneuvers, and 1,000 driver eye heights. Field tests were conducted under a variety of geo- metric, weather, and surprise conditions; under closed-course and open-roadway conditions; and with and without antilock braking systems. From the results of those field studies, they determined that 2.5 s was the 90th percentile value for perception–reaction time (PRT) and that 3.4 m/s2 (11.2 ft/s2) was the 10th percentile deceleration rate. In addition, they identified 1080 mm (3.5 ft) as the 10th percentile driver eye height and 600 mm (2.0 ft) as the 10th percentile object height. Using these as design values, they recommended revised SSDs for design as shown in Table 1. Based on those distances, the authors also recommended new design controls for vertical curves, reproduced in Table 2. Wang (2007) analyzed the placement design of ramp control signals in relation to the satisfaction of a driver’s comfortable cone of vision for stopped vehicles at the stop line and the satisfaction of SSD for approaching vehicles in accordance with MUTCD. He derived relationships between location of stop line, location of signal standard, ramp geometry, and approaching speeds, and he developed sample lookup design charts to facilitate the development and evaluation of signal placement design. A brief analysis using these relationships concluded that signal standards placed in alignment with the stop line would violate not only the comfortable cone of vision of stopped drivers but also the SSD of approaching vehicles. He concluded that for a loop on-ramp with a 300-ft radius, standard-mounted signals on the left side of the ramp should be placed at least 22 ft downstream of the stop line to satisfy the requirements of both the stopped and approaching vehicles. In contrast, he added, signals on the right side of the loop ramp could satisfy only the stopped vehicles but not the approaching vehicles if placed at least 44 ft downstream of the stop line. Therefore, for a loop ramp with a smaller radius (approximately 300 ft or less), two signal indications are needed to satisfy the MUTCD’s requirements, with one at the left side of the ramp curve to provide sufficient sight distance for the approaching vehicles and one at the right side of the ramp curve to provide sufficient viewing angle for the stopped vehicles. He added that signals placed on the left side of the on-ramp curve of a loop ramp (even with a radius greater than 300 ft) are more critical than those on the right side, especially when the approaching SSD is important. Sarhan and Hassan (2008) sought to develop a reliability- based probabilistic approach that was well suited to replace deterministic highway design practice. In their study, reliabil- ity analysis was used to estimate the POH that might result from insufficiency of SSD. As an application, they checked the available sight distance against the required SSD on an assumed road segment. Variation of the design parameters was addressed with Monte Carlo simulation using 100,000 sets of design parameters based on distributions available in the literature. They also developed a computer program to use these sets of design parameters to calculate the profiles of available and required SSD in 2- and 3-D projections as well as the profile of POH. They applied their approach to a horizontal curve with 100-km/h (62-mph) design speed overlapping with flat grade, crest curves, and sag curves in a cut section where the side slope would restrict the sightline. chapter three elementS Of DeSign

14 They determined that their analysis showed that the current deterministic approach yielded very conservative estimates of available and required SSD, resulting in very low POH (0.302%). An application example also showed the change of POH with the change of vertical alignment parameters. They concluded that although changes in vertical alignment caused a significant change in POH in relative terms, the absolute value of POH remained low, indicating that current design practice may be uneconomical. However, the signifi- cance of the different values of POH in terms of safety impli- cations remains a subject for further investigation. Passing Sight Distance Carlson et al. (2005) investigated characteristics of daytime high-speed passing maneuvers along a straight and flat 15-mi section of a rural two-lane, two-way highway. The posted speed limit on this highway in Texas was 70 mph, and the researchers recorded characteristics of passing maneuvers from their own vehicle, which was driven at speeds of 55, 60, and 65 mph to encourage passing by adjacent drivers. They recorded 105 single-vehicle daytime passing maneuvers, and they developed speed profiles of the passing vehicles for each of the three studied speeds. The researchers then compared their findings with AASHTO’s assumptions and criteria for minimum PSD for two-lane, two-way highways. In particu- lar, their analysis focused on the elements associated with a passing vehicle while it occupied the opposing lane of travel. The specific elements that were studied included average passing speed, speed differential between passing and passed vehicles, distance traveled while making the pass, and total elapsed time. Their general findings provided support for the AASHTO PSD model, and the researchers concluded that the model provided reasonable results for the assumptions made. However, they added, the assumptions may need to be updated or have more flexibility added. For instance, for a 70 mph design speed, the assumed speed of the overtaken vehicle was 54 mph in the AASHTO PSD model, which was verified in this study. However, they also concluded that the then-current AASHTO PSD model would provide inadequate PSD values for speeds of overtaken vehicles that were greater than those assumed (e.g., 60 or 65 mph). Under NCHRP Project 15-26, Harwood et al. (2008) evalu- ated current methods for determining minimum PSD require- ments. Based on their results, the research team assessed the guidance on PSD provided in the Green Book and the MUTCD. The assessment considered safety concerns on two-lane high- TABlE 1 RECOMMENDED STOPPINg SIgHT DISTANCES FOR DESIgN Initial Speed Perception-Brake Reaction Deceleration Braking Distance Stopping Sight Distance for Design Time, s Distance km/h mph m ft m/s2 ft/s2 m ft m ft 30 18.6 2.5 20.8 68.2 3.4 11.2 10.2 33.5 31.0 101.7 40 24.9 2.5 27.8 91.2 3.4 11.2 18.2 59.7 45.9 150.6 50 31.1 2.5 34.7 113.8 3.4 11.2 28.4 93.2 63.1 207.0 60 37.3 2.5 41.7 136.8 3.4 11.2 40.8 133.9 82.5 270.7 70 43.5 2.5 48.6 159.4 3.4 11.2 55.6 182.4 104.2 341.9 80 49.7 2.5 55.6 182.4 3.4 11.2 72.6 238.2 128.2 420.6 90 55.9 2.5 62.5 205.1 3.4 11.2 91.9 301.5 154.4 506.6 100 62.1 2.5 69.4 227.7 3.4 11.2 113.5 372.4 182.9 600.1 110 68.4 2.5 76.4 250.7 3.4 11.2 137.3 450.5 213.7 701.1 120 74.6 2.5 83.3 273.3 3.4 11.2 163.4 536.1 246.7 809.4 Source: Fambro et al. (2000). TABlE 2 RECOMMENDED DESIgN CONTROlS FOR VERTICAl CURVES Initial Speed Stopping Sight Distance for Design Rate of Vertical Curvature, K (length per % of algebraic difference in grade A) Crest Curves Sag Curves km/h mph m ft m ft m ft 30 18.6 31.0 101.7 2 6.6 5 16.4 40 24.9 45.9 150.6 4 13.1 8 26.2 50 31.1 63.1 207.0 7 23.0 12 39.4 60 37.3 82.5 270.7 11 36.1 17 55.8 70 43.5 104.2 341.9 17 55.8 23 75.5 80 49.7 128.2 420.6 25 82.0 29 95.1 90 55.9 154.4 506.6 37 121.4 37 121.4 100 62.1 182.9 600.1 51 167.3 45 147.6 110 68.4 213.7 701.1 70 229.7 53 173.9 120 74.6 246.7 809.4 93 305.1 62 203.4 Source: Fambro et al. (2000).

15 ways, driver behavior, and the possible influence of longer trucks and older drivers. The findings of the research, docu- mented in NCHRP Report 605, presented recommendations to bring consistency between PSD design standards and pavement marking practices. Researchers recommended that the MUTCD PSD criteria used for marking of passing and no-passing zones on two-lane roads be used in PSD design in the AASHTO Green Book. The support for the decision was that, in addition to providing the desired consistency between PSD design and marking practices, the research found that two-lane highways could be designed to oper- ate safely with the MUTCD criteria. The researchers added that the longer PSD criteria presented in the then-current edition of the Green Book might provide improved traffic operational efficiency, but they were often considered to be impractical. Their recommendations for PSD design values are presented in Table 3. hOrizOntal alignment Bidulka et al. (2002) investigated the effect of overlapping ver- tical alignment on the horizontal curvature perceived by the driver. Initially, their hypothesis was “that overlapping crest curves made the horizontal curvature appear sharper and over- lapping sag curves made the horizontal curvature appear less sharp.” Researchers noted those drivers’ responses to both static and dynamic computer-generated 3-D images of roadways indicated that this hypothesis was valid and that it was more evident in the case of sag curves. They concluded that errone- ous perceptions, as influenced by vertical curves, increased as (1) the sight distance increased, (2) the horizontal curve radius increased, and (3) the length of vertical curve per 1% change in grade decreased. They reported that driver characteristics did not appear to affect the horizontal curve perception. Hassan et al. (2002) continued the experiment in an attempt to quantify the extent of the driver’s erroneous perception. Drivers were shown an image of a horizontal curve overlap- ping a vertical curve (test curve) and a number of horizon- tal curves overlapping flat vertical grades and with different radii (reference curves), as shown in Figure 2. The driver was then asked to state which reference curve had a radius closest to that of the test curve. Analysis of drivers’ responses led the researchers to conclude that drivers would drive faster on horizontal curves in sag combinations and slower on horizontal curves in crest combinations. They added that the approach tangent, generally a downgrade in sag combina- tions and an upgrade in crest combinations, would further encourage the tendency to drive faster on sag combinations and slower on crest combinations. They recommended that designers establish the profile and predicted operating speed of an alignment based on a 3-D model, rather than a traditional 2-D model. lamm et al. (2002) developed a process to evaluate the safety of horizontal alignment on two-lane rural roads. The primary parameter in their methodology was the change in curvature rate of a given curve, which they tested against several databases of accident rates and accident cost rates and found to be a major descriptor of safety. They developed quantitative ranges for three safety criteria (design consis- tency, operating speed consistency, and driving dynamic con- sistency) and associated them with design classes for good, fair, and poor practices with respect to accidents. They con- cluded that the use of their methodology would allow design- ers to predict the potential accident risks and safety problems of a particular alignment and make changes to remedy them or develop countermeasures to mitigate them. Schurr et al. (2002) studied circular horizontal curves on rural two-lane highways in Nebraska to determine rela- tionships among design speed, operating speed, and posted speeds for developing horizontal alignment design guide- lines. They found that for drivers at their study sites [curves TABlE 3 PASSINg SIgHT DISTANCE FOR DESIgN OF TWO-lANE HIgHWAyS Metric U.S. Customary Design Speed (km/h) Assumed Speed of Passed Vehicle (km/h) Assumed Speed of Passing Vehicle (km/h) Passing Sight Distance (m) Design Speed (mph) Assumed Speed of Passed Vehicle (mph) Assumed Speed of Passing Vehicle (mph) Passing Sight Distance (ft) 30 11 30 120 20 8 20 400 40 21 40 140 25 13 25 450 50 31 50 160 30 18 30 500 60 41 60 180 35 23 35 550 70 51 70 210 40 28 40 600 80 61 80 245 45 33 45 700 90 71 90 280 50 38 50 800 100 81 100 320 55 43 55 900 110 91 110 355 60 48 60 1,000 120 101 120 395 65 53 65 1,100 130 111 130 440 70 58 70 1,200 75 63 75 1,300 80 68 80 1,400 Source: Harwood et al. (2008).

16 with radii greater than or equal to 350 m (1,146 ft)], as the deflection angle increased, speed measures (mean, 85th percentile, and 95th percentile) decreased. They concluded from this finding that motorists may view a large change in direction as a motivation to slow their speed. They also found that as curve length increased, their speed measures increased, leading them to conclude that drivers were moti- vated to increase their speed as a curve lengthens, suggesting they may become more comfortable at higher speeds because they have more time to adjust their vehicle path to a constant radius. They also concluded that grade has an influence on the upper-percentage range of vehicle speeds, because the 85th percentile speed decreased as approach grade increased at their study sites. Finally, they found that as ADT increased, 95th percentile speed decreased; the authors speculated that roadways with higher ADT values may be perceived by drivers as having a higher likelihood of speed enforcement. The report contains their recommendation for using a series of equations to estimate mean, 85th percentile, and 95th percen- tile operating speeds at approach locations and midpoints of horizontal curves in Nebraska or curves with similar charac- teristics. Those equations are shown in Table 4. Schurr et al. (2005) also developed a model to describe design speed profiles of vehicles traversing horizontal curves on approaches to stop-controlled intersections on two-lane RETURN TO [PHASE CHOICES] [RETURN TO HOME] RETURN TO [PHASE II ANIMATIONS] [Back] [Next] From the list of roads on the right, pick the one that has a curve most similar to the road on the left. FIGURE 2 Presentation of horizontal curve images for comparison (Hassan et al. 2002). TABlE 4 EqUATIONS FOR ESTIMATINg OPERATINg SPEEDS AT HORIzONTAl CURVES Speed Approach Location Midpoint of Curve Mean V = 51.7 + 0.508 Vp V = 67.4 − 0.1126∆ + 0.02243 L + 0.276 Vp 85th Percentile V = 70.2 + 0.434 Vp – 0.001307 TADT V = 103.3 – 0.1253∆ + 0.0238 L + 1.039 G1 95th Percentile V = 84.4 + 0.352Vp – 0.001399 TADT V = 113.9 – 0.122∆ + 0.0178 L + 0.00184 TADT Source: Schurr et al. (2002). Notes: V = speed (km/h) of free-flow passenger cars at that location; Vp = posted speed limit (km/h); TADT = traffic volume (vehicles/day); ∆ = deflection angle (decimal degrees); L = arc length of curve (m); G1 = approach grade (percent).

17 two-way rural highways. They used the model to create a pro- cedure for designing horizontal curves that would accommo- date vehicles transitioning from high speeds to a stop. Based on speed profile data from 15 study sites in Nebraska, the researchers concluded that posted speed, median type, pres- ence of rumble bars, roadway surface condition, and degree of rutting did not significantly affect the vehicle speed profiles at these sites at a 95% confidence level. They also concluded that the intercepts of the regression lines for approaches with and without horizontal curves were significantly different in the case of heavy vehicles. The speed of heavy vehicles on tan- gent approaches was generally about 8 mph higher than on sites that exhibited horizontal curvature, although the rate of decel- eration remained almost the same until vehicles were near the stop. Passenger cars exhibited no statistically significant differ- ence between curved and tangent alignments. Researchers used the results of the study to develop a procedure for determining the minimum curve radius appropriate for a roadway alignment approaching a stop ensuring that (1) the visual expectations of the driver were met, (2) the comfort of the passengers within the vehicle was optimized, (3) the curve design used a simple curve with no spirals, (4) the vehicle speed within the limits of the curve were reasonable, (5) sufficient braking distance to the stop was available, and (6) deceleration rates were reasonable. Cafiso et al. (2005b) sought to determine design inconsis- tencies on existing two-lane rural roads in Italy with the use of actual driving behavior and to verify their agreement with a consistency evaluation model. They developed a data col- lection method using a sample of test drivers operating an instrumented vehicle on a pre-determined route. From this study they concluded the following: • A coordinate sequence of curves did not produce an unexpected driving event even if short bending radii were adopted. • geometric inconsistency produced by a sharp curve following a long tangent produced tense driving behav- ior, as observed on curves with radii of 120 m (394 ft) and 80 m (262 ft). • Driving inconsistencies were highlighted by high- speed gradients of about 2 m/s2 (6.5 ft/s2), transversal accelerations of 0.3 g, and local maximum curvatures of the car path higher than those required by horizontal alignment. These values of deceleration reached with a light braking action were higher than the 0.80 to 0.85 m/s2 (2.62 to 2.79 ft/s2) generally assumed with regard to driving behavior in speed profile diagrams. • Maneuvers were caused by the driver’s need to suddenly correct his or her driving behavior owing to an unexpected alignment and could produce a dangerous situation if bad pavement conditions or unexpected events occur. • The lack of transition curves was also a contributing factor in geometric inconsistency. lyles and Taylor (2006) stated that, “historically, the hor- izontal curve is the most critical geometric design element that influences driver behavior and has the most potential for crashes.” They added that “research has indicated that the average accident rate for horizontal curves is about three times the average accident rate for highway tangents and the average run-off-the-road crash rate for highway curves is about four times that of highway tangents.” They stated that many curve-related crashes were the result of drivers approaching and entering the curve at a speed that was too fast for the alignment. A study of driver behavior and errors on a selection of horizontal curves led them to conclude the following: • Drivers approaching curves routinely exceeded the posted speed limit as well as the posted advisory speed, where applicable. • Drivers had more errors at curves where they had lim- ited or no visibility of the curves when the traffic control devices (TCDs) were first visible. • Drivers made more errors on horizontal curves that were adjacent to vertical curves, particularly crests that obscured a downstream horizontal curve. • There were increased errors when curves were com- bined with other elements, especially intersections. Many design standards recommend the use of spiral curves in the transition design. Perco (2006) conducted a study to evaluate effects of a long spiral transition on the driver’s curve perception and safety. He analyzed driving paths on 12 transitions with and without spiral curves, and concluded that the results confirmed a negative effect of excessive spiral length on driver behavior. His analysis results showed that the most desirable spiral length, which offered advantages in comparison with a tangent-to-curve transition, was equal to the distance traveled during the steering time. He developed a model to estimate the desirable spiral length for transitions of sharp horizontal curves on two-lane rural roads, based on the data collected in three studies. Starting from the radius of the impending curve, the model calculated the desirable spiral length and provided a description of actual driver behavior, as observed in field surveys. Perco concluded that the choice of the spiral length based on this model was useful because the estimated length was consistent with the real distance traveled by the vehicle during the steering action, which ensured opti- mal operating conditions for drivers. vertical alignment Hassan (2004) described the development of two models to determine the required SSD on crest and sag vertical curves. By comparing profiles of available SSD and required SSD on examples of vertical curves, Hassan concluded that current North American design practices might yield segments of the vertical curve where the driver’s view is constrained to a dis- tance shorter than the required SSD. He developed new models based on longitudinal friction and on acceleration, then devel- oped an alternative design procedure based on the models,

18 which he used to determine recommendations for minimum lengths of crest and sag vertical curves. Depending on the approach grade, the new values of minimum curve length could be greater than or less than values obtained through conventional design procedures; design aids were therefore provided in tabular form to facilitate use by designers. Torbic et al. (2005) conducted a study to determine the distribution of truck weight/power ratios in the current truck fleet in several regions of the United States, and compare them with the 120 kg/kW (200 lb/hp) value recommended in the 2001 Green Book. The researchers collected data on truck crawl speeds at locations in California, Colorado, and Pennsylvania and concluded that a “weight/power ratio of 102 to 108 kg/kW (170 to 180 lb/hp) would be appropriate for freeways in California and Colorado, and a weight/power ratio of 126 kg/kW (210 lb/hp) would be more appropriate in Pennsylvania.” They also determined that truck performance on two-lane highways was sufficiently different from free- ways to recommend different ratios for those roads: a 108 kg/kW (180 lb/hp) design vehicle in Colorado, and 150 to 168 kg/kW (250 to 280 lb/hp) for California and Pennsyl- vania. According to the researchers, all of these ratios rep- resented the 85th percentile of the truck population that was studied; therefore, most of the truck population performed substantially better. Motivated by changes in headlamp design in recent decades, Hawkins and gogula (2008) reviewed existing sag curve design criteria to determine if revisions to the design procedure were appropriate. They compared theoretical and field measurements of the levels of illuminance falling across the road surface, provided by sealed-beam and modern head- lamps, as illustrated in Figure 3. The results of their analysis indicated that modern headlamps provided significantly less light above the horizontal than sealed-beam headlamps, indi- cating a potential need to modify the design equations for sag vertical curves. According to their theoretical analysis, the upward divergent headlamp angle used in the sag curve design equation should be reduced from 1° to between 0.75° and 0.90°. They stated that results from field analysis indi- cated a significant difference in illuminance levels from the theoretical analysis, but also indicated a need to reduce the headlamp angle used in sag curve design. Easa (2008) developed a single-arc unsymmetrical verti- cal curve that takes the form of a cubic instead of parabolic function. The curve has a rate of change in grade that gradu- ally varies between the start and end of the vertical curve, which eliminates the sudden change in curvature of tradi- tional two-arc unsymmetrical vertical curves. He developed sight distance relationships for the new single-arc crest curve, which established the sight distance profile for the new curve and shows a substantial improvement over the abrupt-type sight distance profiles of two-arc curves. Included in the description of the single-arc curve characteristics are length requirements to satisfy AASHTO stopping, passing, and deci- sion sight distance guidelines. The Highway Safety Manual (AASHTO 2010) provides guidance on the effect of grades on expected safety of road- way segments. The base condition for grade is a generally level roadway. Table 5 presents the crash modification factors (CMFs) for grades based on an analysis of rural two-lane, two-way highway grades in Utah. The CMFs in the table are applied to each individual grade segment on the roadway being Horizontal Plane lane width left point center point right point g2(actual grade is immaterial) h + S tan α h L (L>S) Vertical Plane g1 Hh α=Hv S FIGURE 3 Illustration of headlamp analysis approach (Hawkins and Gogula 2008). TABlE 5 CRASH MODIFICATION FACTORS FOR gRADE OF ROADWAy SEgMENTS Approximate Grade (%) Level Grade (≤3%) Moderate Terrain (3% < grade ≤ 6%) Steep Terrain (>6%) CMF 1.00 1.10 1.16 Source: AASHTO (2010).

19 evaluated without respect to the sign of the grade. The sign of the grade is irrelevant because each grade on a rural two-lane, two-way highway is an upgrade for one direction of travel and a downgrade for the other. The grade factors are applied to the entire grade from one point of vertical intersection to the next. The CMFs apply to total roadway segment crashes. wOrk zOne cOnSiDeratiOnS NCHRP Report 581 (Mahoney et al. 2004) discusses sev- eral elements of design and their relation to work zones. The authors state that although extended sight distances through- out work zones are desirable, the underlying need for decision sight distance (because of an unexpected or difficult-to- perceive information source or condition) should be avoided in designing construction work zones. Temporary traffic control and other driver information strategies are used in conjunction with extended sight distance to mitigate work zone condi- tions that are atypical or involve complex driver decisions. They concluded “that extended sight distance approach- ing and within work zones is desirable from an operations perspective. Safety issues also point to [the need for] some minimum sight distance.” For work zone design speeds less than 40 mph, the SSD values tabulated in the Green Book and corresponding to work zone design speed were recommended. For work zone design speeds of 40 mph and greater, the Green Book design-speed-corresponding values did not necessarily represent the minimum values that could be accepted; a minimum sight distance of 300 ft was rec- ommended using a driver eye height of 3.5 ft and an object height of 2.0 ft. Maximum superelevation rates (emax) are typically selected as a matter of policy rather than for specific proj- ects. Absent other considerations, the emax used for perma- nent roadways is appropriate for construction work zones. Superelevating roadway curves necessitates superelevation transitions, which bring alignment and other (e.g., drainage) complications. For these reasons, it is common design prac- tice to provide curves that are sufficiently flat to not require the introduction of superelevation. Mahoney et al. (2004) discuss the use of Methods 2 and 5 from the Green Book for determining appropriate superelevation distributions. NCHRP Report 581 (Mahoney et al. 2004) also states that, in general, the same maximum grade criteria applicable to the highway under construction should be applied to work zone roads. However, marginally exceeding these criteria is often justified in consideration of all factors. grades below the maximum are desirable. When designing work zone tem- porary roadways, the potential effect of grades on operations and capacity should be considered. When speeds are sub- stantially reduced in advance of a temporary roadway (e.g., in conjunction with a reduction in the number of lanes), the work zone capacity may be controlled by heavy vehicles attempting to accelerate on grade, which, in turn, influences queue formation. The authors found that the most common basis for agency work zone sight distance design criteria is the set of SSD values from the Green Book. For this case, mini- mum crest and sag vertical curve lengths are determined from Exhibits 3-71 and 3-74, respectively, in the 2004 Green Book. Summary Of key finDingS This section summarizes key findings from the research noted in this chapter. This is an annotated summary; conclu- sions and recommendations are those of the authors of the references cited. Stopping Sight Distance • New values for SSD and new design controls for verti- cal curves were recommended, based on a PRT of 2.5 s, a 10th percentile deceleration rate of 11.2 ft/s2, a 10th percentile driver eye height of 3.5 ft and a 10th percen- tile object height of 2.0 ft (Fambro et al. 2000). • Ramp control signals placed on the left side of a curve of a loop on-ramp (even with a radius greater than 300 ft) are more critical for accommodating SSD than those on the right side (Wang 2007). • The method of selecting SSD values deterministically yielded conservative estimates of available and required SSD, resulting in a very low probability (0.302%) of hazard (Sarhan and Hassan 2008). Passing Sight Distance • An analysis of observed passing maneuvers provided support for the AASHTO PSD model, and the model provided reasonable results for the assumptions made. However, the model’s assumptions may need to be updated or accommodate more flexibility for speeds higher than 55 mph (Carlson et al. 2005). • Increased consistency between AASHTO PSD design standards and MUTCD pavement marking practices was recommended, specifically accomplished by using the MUTCD criteria for marking passing/no-passing zones on two-lane roads in the Green Book’s PSD design pro- cess. In addition to providing the desired consistency between PSD design and marking practices, two-lane highways could be designed to operate safely with the MUTCD criteria (Harwood et al. 2008). horizontal alignment • Erroneous perceptions by drivers approaching horizon- tal curves, as influenced by vertical curves, increased as (1) the sight distance increased, (2) the horizontal curve radius increased, and (3) the length of vertical curve per 1% change in grade decreased. Drivers tend to drive faster on horizontal curves in sag combinations and slower on horizontal curves in crest combinations.

20 Designers can establish the profile and predicted oper- ating speed of an alignment based on a 3-D model, rather than a traditional 2-D model (Bidulka et al. 2002; Hassan et al. 2002). • For drivers on curves with radii greater than or equal to 350 m (1,146 ft), as the deflection angle increased, speed measures (mean, 85th percentile, and 95th percentile) decreased; as a result, motorists may view a large change in direction as a motivation to slow their speed. In addi- tion, as curve length increased, speed measures increased, suggesting that drivers may become more comfortable at higher speeds because they have more time to adjust their vehicle path to a constant radius. grade has an influ- ence on the upper-percentage range of vehicle speeds, because the 85th percentile speed decreased as approach grade increased (Schurr et al. 2002). • A study of driver behavior and errors on a selection of horizontal curves led lyles and Taylor (2006) to con- clude the following: – Where applicable, drivers approaching curves rou- tinely exceeded the posted speed limit as well as the posted advisory speed. – Drivers had more errors at curves where they had limited or no visibility of the curves when the TCDs were first visible. – Drivers made more errors on horizontal curves that were adjacent to vertical curves, particularly crests that obscured a downstream horizontal curve. – There were increased errors when curves were com- bined with other elements, especially intersections. vertical alignment • Current North American design practices might yield segments of the vertical curve where the driver’s view is constrained to a distance shorter than the required SSD. An alternative design procedure is recommended based on a new model that incorporated longitudinal friction and acceleration, which produced new recom- mended values for minimum lengths of crest and sag vertical curves (Hassan 2004). • A weight/power ratio of 102 to 108 kg/kW (170 to 180 lb/hp) would be appropriate for freeways in California and Colorado, and a weight/power ratio of 126 kg/kW (210 lb/hp) would be more appropriate in Pennsylvania, as compared with the 120 kg/kW (200 lb/hp) value rec- ommended in the 2001 Green Book (Torbic et al. 2005). • The upward divergent headlamp angle used in the sag curve design equation should be reduced from 1° to between 0.75° and 0.90° (Hawkins and gogula 2008).

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 432: Recent Roadway Geometric Design Research for Improved Safety and Operations reviews and summarizes roadway geometric design literature completed and published from 2001 through early 2011, particularly research that identified impacts on safety and operations.

The report is structured to correspond to chapters in the American Association of State Highway and Transportation Officials’ A Policy on Geometric Design of Highways and Streets, more commonly referred to as the Green Book.

NCHRP Synthesis 432 is an update of NCHRP Synthesis 299 on the same topic published in 2001.

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