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

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14 Table 1 Recommended Stopping Sight Distances for Design Stopping Sight Perception-Brake Reaction Distance for Initial Speed Distance Deceleration Braking Distance Design km/h mph Time, s 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). They determined that their analysis showed that the current their findings with AASHTO's assumptions and criteria for deterministic approach yielded very conservative estimates minimum PSD for two-lane, two-way highways. In particu- of available and required SSD, resulting in very low POH lar, their analysis focused on the elements associated with a (0.302%). An application example also showed the change passing vehicle while it occupied the opposing lane of travel. of POH with the change of vertical alignment parameters. The specific elements that were studied included average They concluded that although changes in vertical alignment passing speed, speed differential between passing and passed caused a significant change in POH in relative terms, the vehicles, distance traveled while making the pass, and total absolute value of POH remained low, indicating that current elapsed time. Their general findings provided support for the design practice may be uneconomical. However, the signifi- AASHTO PSD model, and the researchers concluded that cance of the different values of POH in terms of safety impli- the model provided reasonable results for the assumptions cations remains a subject for further investigation. 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 Passing Sight Distance vehicle was 54 mph in the AASHTO PSD model, which was Carlson et al. (2005) investigated characteristics of daytime verified in this study. However, they also concluded that the high-speed passing maneuvers along a straight and flat 15-mi then-current AASHTO PSD model would provide inadequate section of a rural two-lane, two-way highway. The posted PSD values for speeds of overtaken vehicles that were greater speed limit on this highway in Texas was 70 mph, and the than those assumed (e.g., 60 or 65 mph). researchers recorded characteristics of passing maneuvers from their own vehicle, which was driven at speeds of 55, 60, Under NCHRP Project 15-26, Harwood et al. (2008) evalu- and 65 mph to encourage passing by adjacent drivers. They ated current methods for determining minimum PSD require- recorded 105 single-vehicle daytime passing maneuvers, and ments. Based on their results, the research team assessed the they developed speed profiles of the passing vehicles for each guidance on PSD provided in the Green Book and the MUTCD. of the three studied speeds. The researchers then compared The assessment considered safety concerns on two-lane high- Table 2 Recommended Design Controls for Vertical Curves Rate of Vertical Curvature, K (length per % of algebraic difference in grade A) Stopping Sight Initial Speed Distance for Design 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).