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15 Table 3 Passing Sight Distance for Design of Two-Lane Highways Metric U.S. Customary Assumed Assumed Assumed Assumed Speed of Speed of Speed of Speed of Design Passed Passing Passing Sight Design Passed Passing Passing Sight Speed Vehicle Vehicle Distance Speed Vehicle Vehicle Distance (km/h) (km/h) (km/h) (m) (mph) (mph) (mph) (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). ways, driver behavior, and the possible influence of longer Drivers were shown an image of a horizontal curve overlap- trucks and older drivers. The findings of the research, docu- ping a vertical curve (test curve) and a number of horizon- mented in NCHRP Report 605, presented recommendations tal curves overlapping flat vertical grades and with different to bring consistency between PSD design standards and radii (reference curves), as shown in Figure 2. The driver was pavement marking practices. Researchers recommended that then asked to state which reference curve had a radius closest the MUTCD PSD criteria used for marking of passing and to that of the test curve. Analysis of drivers' responses led no-passing zones on two-lane roads be used in PSD design the researchers to conclude that drivers would drive faster in the AASHTO Green Book. The support for the decision on horizontal curves in sag combinations and slower on was that, in addition to providing the desired consistency horizontal curves in crest combinations. They added that the between PSD design and marking practices, the research approach tangent, generally a downgrade in sag combina- found that two-lane highways could be designed to oper- tions and an upgrade in crest combinations, would further ate safely with the MUTCD criteria. The researchers added encourage the tendency to drive faster on sag combinations that the longer PSD criteria presented in the then-current and slower on crest combinations. They recommended that edition of the Green Book might provide improved traffic designers establish the profile and predicted operating speed operational efficiency, but they were often considered to be of an alignment based on a 3-D model, rather than a traditional impractical. Their recommendations for PSD design values 2-D model. are presented in Table 3. Lamm et al. (2002) developed a process to evaluate the Horizontal Alignment safety of horizontal alignment on two-lane rural roads. The primary parameter in their methodology was the change in Bidulka et al. (2002) investigated the effect of overlapping ver- curvature rate of a given curve, which they tested against tical alignment on the horizontal curvature perceived by the several databases of accident rates and accident cost rates driver. Initially, their hypothesis was "that overlapping crest and found to be a major descriptor of safety. They developed curves made the horizontal curvature appear sharper and over- quantitative ranges for three safety criteria (design consis- lapping sag curves made the horizontal curvature appear less tency, operating speed consistency, and driving dynamic con- sharp." Researchers noted those drivers' responses to both static sistency) and associated them with design classes for good, and dynamic computer-generated 3-D images of roadways fair, and poor practices with respect to accidents. They con- indicated that this hypothesis was valid and that it was more cluded that the use of their methodology would allow design- evident in the case of sag curves. They concluded that errone- ers to predict the potential accident risks and safety problems ous perceptions, as influenced by vertical curves, increased as of a particular alignment and make changes to remedy them (1) the sight distance increased, (2) the horizontal curve radius or develop countermeasures to mitigate them. increased, and (3) the length of vertical curve per 1% change in grade decreased. They reported that driver characteristics did Schurr et al. (2002) studied circular horizontal curves not appear to affect the horizontal curve perception. on rural two-lane highways in Nebraska to determine rela- tionships among design speed, operating speed, and posted Hassan et al. (2002) continued the experiment in an attempt speeds for developing horizontal alignment design guide- to quantify the extent of the driver's erroneous perception. lines. They found that for drivers at their study sites [curves

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16 [RETURN TO HOME] RETURN TO [PHASE CHOICES] 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). with radii greater than or equal to 350 m (1,146 ft)], as the at their study sites. Finally, they found that as ADT increased, deflection angle increased, speed measures (mean, 85th 95th percentile speed decreased; the authors speculated that percentile, and 95th percentile) decreased. They concluded roadways with higher ADT values may be perceived by drivers from this finding that motorists may view a large change as having a higher likelihood of speed enforcement. The in direction as a motivation to slow their speed. They also report contains their recommendation for using a series of found that as curve length increased, their speed measures equations to estimate mean, 85th percentile, and 95th percen- increased, leading them to conclude that drivers were moti- tile operating speeds at approach locations and midpoints of vated to increase their speed as a curve lengthens, suggesting horizontal curves in Nebraska or curves with similar charac- they may become more comfortable at higher speeds because teristics. Those equations are shown in Table 4. they have more time to adjust their vehicle path to a constant radius. They also concluded that grade has an influence on Schurr et al. (2005) also developed a model to describe the upper-percentage range of vehicle speeds, because the design speed profiles of vehicles traversing horizontal curves 85th percentile speed decreased as approach grade increased on approaches to stop-controlled intersections on two-lane 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 V = 103.3 0.1253 + 0.0238 L + 1.039 G1 TADT V = 84.4 + 0.352Vp 0.001399 TADT V = 113.9 0.122 + 0.0178 L + 0.00184 95th Percentile 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).