However, many agencies are designing intersections along their primary systems to accommodate a 70 ft, single trailer design vehicle (C-70). Knoblauch, et al. Several participants suggested that a combination of roadside and overhead signs, in addition to roadway markings, would be beneficial. A total of 134 participants were tested in three age groups: 18 to 29, 30 to 59, and 60 and above. Before the roundabout, the crashes were almost all angle crashes, and after the roundabout was installed, one of the crashes was a single-vehicle crash against a fixed object, and the other crash was a rear-end crash. In Case I, the PRT is assumed to be 2.0 s plus an additional 1.0 s to actuate braking, although the "preferred design" uses stopping sight distance (SSD) as the ISD design value (which incorporates a PRT of 2.5 s). 80) to provide general guidance to project engineers on appropriate applications, site requirements, geometric elements, and traffic analysis. Design Speed (km/h) Decision Sight Distance (meters) Stop Rural Road: Stop Urban Compliance with traffic control devices was found to be poor for all pedestrians at all study locations; it was also found that most pedestrians keyed on the moving vehicle rather than on the traffic and pedestrian control devices. Data for each roundabout is reported inTable 35. Alexander (1989) proposed the addition of a 'search time" variable to the current equations for determining ISD, and use of the PRT value currently employed in the SSD computations (i.e., 2.5 s) for all ISD computations. (2007). Table 30, from Janoff (1990), presents the peak intensity requirements of red, green, and yellow traffic signals for 200-mm (8-in) signals for normal-speed roads and for 12-in signals for high-speed roads; the values presented exclude the use of backplates and ignore depreciation. The authors indicate that locating the green arrow to the right of the red-ball indication in a 5-section horizontal display arrangement appears to provide confusion. According to Harkey, et al. This increase in distance reflects 8 s of increased awareness of the crossing for a 30-mph operating speed. The 50th percentile gap acceptance point was about 7 s (i.e., if a gap is 7 s long, only about half of the subjects would accept it). A number of research efforts have been conducted to determine appropriate PRT values for use in ISD computations. Analysis of the North Carolina motor vehicle crash file for 19801990 displayed somewhat smaller percentages, but showed the trend of increasing pedestrian crashes at intersections as age increased. The results found that of the 20 intersection approaches, the proportion of pedestrians entering the street during the flashing or steady hand decreased at 13 of the approaches (6 decreases were significant) with a significant increase at 2 approaches. Both young-old and old-old drivers in this study were most concerned about ease of turning, citing the better maneuverability and less chance of hitting the curb as their primary basis of response. In one study, approximately 85 percent of the pedestrians age 60 and older required a minimum gap of 9 s before crossing the road, while only 63 percent of all pedestrians required this minimum gap size duration (Tobey, Shungman, and Knoblauch, 1983). The results indicated that right-turn channelization affects the speed at which drivers make right turns and the likelihood that they will stop before making a RTOR. In addition, if the number of opposing lanes to be crossed exceeds one, an additional 0.5 s per additional lane for passenger cars and 0.7 s per additional lane for trucks was recommended. The aging eye experiences exaggerated intraocular scatter of lightall light, independent of wavelength (Wooten and Geri, 1987)making these drivers more susceptible to glare. No conclusions could be made regarding the safety effect of replacing the green ball with flashing yellow arrow at the five intersections with permissive phasing. Burnham (1992) noted that the selection of letter size for any sign must evaluate the needs of the user, which are continuously changing as a function of changes in automotive technology, the roadway system, and the population itself. TEH concluded that a significant proportion of drivers do not make a complete stop before executing an RTOR, and a significant portion of drivers do not yield to pedestrians. One of the advantages of using curbed medians and intersection channelization is that it provides a better indication to motorists of the proper use of travel lanes at intersections. Design recommendations for lane width at intersections follow from consideration of vehicle maneuver requirements and their demands on drivers. Vehicles are generally permitted to turn right (or left) on a green light while pedestrians are crossing on the WALK interval. Looking at crash frequencies as a function of mid-block channelization treatment, the raised curb median treatment is associated with the fewest crashes of all three treatment types. They equate the flashing with a vehicle yellow period. However, passive detection also can be used to detect pedestrians within the crosswalk that may need more time to complete their crossing maneuver. A higher percentage of aging pedestrians (compared to other age groups) were found to have difficulties in understanding the conventional pedestrian signal displays. ST-054 (TEH, 2008). Simplified decision process results from one-way operation, yield-at-entry, and a reduced number of conflict points compared to a conventional intersection. Combination with Non-Standard Stopping Sight Distance, 1000 ft (300 m) to 2000 ft (600 m) radius, Exit or entrance downstream along freeway. In this evaluation of 62 lighting and crash studies, 85 percent of the results showed lighting to be beneficial, with approximately one-third of the results statistically significant. how much of the roadway is visible ahead via headlight illumination. For example, Phoenix, Arizona, a city with a large aging driver population, has been using "jumbo" street name signs at signalized intersections since 1973. The results also showed that the through movement produced a lower value than the mean, while the turning maneuvers produced a higher value. Only 7.3 percent of the population had measured walking speeds 3 ft/s, and less than 1 percent had walking speeds of 4.0 ft/s. This compares with 23.7 percent for pedestrians ages 1044 and 13.6 percent for those age 9 and younger. (1991), who conducted laboratory and controlled field studies using 200 younger and older drivers (ages 16 to 70+) to determine the minimum visibility requirements for traffic control devices. Thus, the younger drivers in this study were able to compensate for their impairments, but aging drivers both with and without impairments were unable to make compensations in their (simulated) intersection response selections. The 3- and 4-section displays showing only the green arrow, had average driver response times ranging from 3 s for the under-24 age group to 6 s for the over-65 group. (2007), maximum yaw and maximum forward acceleration were significantly reduced for the improved intersection, for both older and younger drivers, indicating better lateral control of the vehicle, and more stable forward acceleration. However, in designing a new intersection, he stated that the presence of islands is unlikely to offset the disadvantage of large intersection size for the pedestrian. Staplin et al. A study by Allen, O'Hanlon, and McRuer (1977) also concluded that delineation contrast should be maintained above a value of 2.0 for adequate steering performance under clear night driving conditions. ISD = intersection sight distance (feet). The results of the word legibility study conducted during the daytime indicated that the microprismatic sheeting produced a 4 percent improvement in legibility distance, compared to the encapsulated lens sheeting. Janoff also noted that although signal size is included, research performed by Cole and Brown (1968) indicated that signal size is not important because traffic signals are point sources rather than area sources and only intensity affects visibility. (2007) recommended that the supplemental panel bearing the legend "TO TRAFFIC IN CIRCLE" be placed immediately below the R1-2 Yield signs on both sides of the road at the entrance to a roundabout, as shown inFigure 85. He states that the small- to moderate-size roundabouts showed significant reductions in total crashes (from an average annual crash frequency 4.8 to 2.4, or 51 percent) and injury crashes (from an average annual crash frequency of 2.0 to 0.5, or 73 percent). Traffic flow data were collected during 32 field studies in eight cities in four States, and 3-year crash histories for 189 street segments were obtained from cities in two States. (1995) study showed no significant differences in 85th percentile decision/response times between younger and older drivers when subjects were close to the signal at either approach speed. The loss of static and dynamic acuitythe ability to detect fine detail in stationary and moving targetswith advancing age is widely understood. (1996) conducted a survey of 2,129 drivers in five States (California, Minnesota, Mississippi, Pennsylvania, and Texas) to evaluate driver understanding of right-of-way conditions and preference for supplemental signs at two-way, stop-controlled intersections. (1997) to determine the amount of offset that would be required when using the modified 1994 AASHTO model (i.e., J = 2.5 s). These signs are 16 in high and use 8 in capital letters (Rural and Urban Roads, 1973). Regarding channelization for mid-block left-turn treatments, Bonneson and McCoy (1997) evaluated the safety and operational effects of three mid-block left-turn treatments: raised curb medians; two-way, left turn-lanes; and undivided cross sections. The addition of the arrow on the street name sign pointing toward the exit leg showed significantly improved comfort, confidence, and safety over the baseline. However, additional guidance was provided to use a walking speed of 3.0 ft/s at locations where pedestrians are routinely crossing at a slower pace. Before concluding this discussion, certain aspects of YIELD sign operations deserve mention. Drivers were shown photographs of left turn displays from various signalized intersections around the country. Niederhauser, Collins, and Myers (1997) reported the before and after average annual crash history for the five intersections in Maryland that were converted to roundabouts. Harwood, et al. Studies in other countries help to shed some light on the optimum design characteristics of modern roundabouts. There was a small increase in the percentage of pedestrians who left early (i.e., on the flashing hand or solid hand) but that increase was not statistically significant. The high crash rate for protected/permissive phasing may also be a reflection of driver misunderstanding of protected/permissive signal displays. The 95th percentile driver could not be accommodated by the minimum retroreflectivity suggested for the YIELD sign measuring 30 in, for MRVD at both 30 and 55 mph. sight distance cannot be provided. stated that wider medians generally have positive effects on traffic operations and safety; however, wider medians can result in sight restrictions for left-turning vehicles due to the presence of opposite left-turn vehicles. Aging pedestrians wait for longer gaps between vehicles before attempting to cross the road. 163 Exhibit 6-34. The traditional set of transverse parallel lines define the boundaries of a crosswalk for the pedestrian, but they are not particularly visible to approaching drivers, especially in dark and/or wet conditions, compared to other marking patterns. This program calculates the Radius of a Horizontal Curve, using the measured Horizontal Sightline Offset (HSO) and required Stopping Sight Distance (S). At locations where storage capacities for signalized intersections are restricted, or where the queues created by signalized intersections cause operational or safety problems. V = operating speed on the major road (mph). For the permissive indication, the sign appeared to help driver understanding, whereas during the overlap and protected indications it appeared to confuse drivers. One of the most common uses of channelization is for the separation of left-turning vehicles from the through-traffic stream. In particular, it is important to rule out the possibility that the FYA will be (mis)perceived as the timing out of a protected left turn phase, which could actually increase the potential for injurious angle crashes. Hauer (1988) stated that "the standards and design procedures for intersection sight triangles should be modified because there is reason to believe that when a passenger car is taken as the design vehicle, the sight distance is too short for many aging drivers, who take longer to make decisions, move their heads more slowly, and wish to wait for longer gaps in traffic.". At all crosswalk locations, the WALK phase remained a constant 6 s, and the clearance interval was extended from 14 s to 20 s at one intersection 59.7 ft wide, and from 18 to 20 s at the other intersection measuring 79.4 ft wide. Each photograph was taken from approximately the driver's eye location as if the driver were positioned as the first left-turn vehicle in queue in an exclusive left-turn lane. 1997). In addition, the loss of peripheral vision increases an aging pedestrian's chances of not detecting approaching and turning vehicles from the side. Age-related declines (possibly exponential) in the ability to detect angular movement have been reported. Exclusive timing is intended to virtually eliminate turning traffic or other movements that conflict with pedestrians while they cross the street. Looking at the contributing factors in angle and turning collisions for both rural and urban signalized locations, the middle-aged group was much more likely to be characterized by the police officer as having exhibited "no improper driving." As diagrammed inFigure 70, the four right-turn lane geometries were: Figure 70. One of the improved intersections was a signalized intersection with separate signals for each lane, with a leading protected left-turn phase indicated by a green arrow, and redundant upstream signing. For protected/permissive operation, the circular green alone was correctly answered by only 50 percent of the respondents, while the green arrow in combination with the circular green had approximately 70 percent correct responses. Figure 19 is a photo showing a truck about to pass underneath an overhead However, there was a small difference in the crash rate among the last three types of phasing. (Harkey et al., 2008) These results are based on a meta-analysis of 38 studies, including 14 conducted in the U.S. (Elvik and Vaa, 2004) and review by an expert panel as part of the referenced NCHRP study. Given the positive experience of some practitioners who are early adopters of this treatment, further FYA research is a high priority. The number of males and females was approximately equal. ISD for vehicles approaching intersections with no control, at which vehicles are not required to stop, but may be required to adjust speed. Guidelines published by AASHTO (1984) identify depreciation due to dirt on the luminaires and reduced lumen output from the in-service aging of lamps as factors that combine to decrease lighting system performance below design values. Although the difference was statistically significant, Bonneson and McCoy point out that the difference may be too small to be of practical significance. The Iowa signs are on a white background with a route shield and a down arrow pointing to the appropriate approach lane. An empirical Bayes analysis, which included a comparison group to control for trend effects and a reference group to adjust for regression to the mean, indicated a nearly 15% drop in the number of crash claims following introduction of the enhanced-conspicuity backplates. Research findings describing driver performance differences directly affecting the use of pavement markings and delineation focus upon (age-related) deficits in spatial vision. stopping sight distance during daylight conditions, but very short sag The next-best performing signal design was the Modified Backplate. Some of these operations are done simultaneously by many drivers, and some operations, such as shifting gears, may be done before searching for intersecting traffic or may not be required with automatic transmissions. In the Noyce and Kacir (2002) study, age played a significant role in the percentage of correct responses when green arrow and red ball indications were shown simultaneously: for drivers less than 24 years of age, 75 percent of responses were correct, and for drivers over the age of 65, 62 percent of responses were correct. Out of the 50 signalized intersections where the flashing yellow arrow was introduced, in 5 of the locations the phasing was permissive and the round green was replaced by flashing yellow arrow. Crash analyses have shown that aging drivers, ages 5675 and age 76 and older, are overinvolved in left-turn maneuvers at signalized intersections, with failure to yield right-of-way or disregarding the signal the principal violation types (Staplin and Lyles, 1991; Council and Zegeer, 1992). The presence of large commercial signs near intersections has been associated with a significant increase in crashes at stop-controlled intersections (Holahan, 1977). (1992): (1) a 2-ft positive offset provides unrestricted sight distance when the opposite left-turn vehicle is a passenger car, and (2) a 3.5-ft positive offset provides unrestricted sight distance when the opposite left-turn vehicle is a truck, for design speeds up to 70 mph. G = the specified critical gap (in seconds); equal to 5.5 s for crossing one opposing lane plus an additional 0.5 s for each additional opposing lane. In an observational field study conducted as a part of the same project, Staplin et al. ISD for a vehicle on a yield-controlled approach on the minor road to cross the major road. (1997) conducted an observational field study of driver performance as a function of left-turn lane geometry and driver age at four locations where the main road operating speed was 35 mph. The CIE (1988), however, recommends that all signals use backplates of a size (width) of three times the diameter of the signal. Section 2D.03 further states that all messages, borders, and legends on guide signs shall be retroreflective, and all backgrounds shall be retroreflective or illuminated.) Although specific results were not differentiated by age, Taoka asserted that 85th percentile glance times at signs (about 2.4 s) were likely too long, as 2.0 s is the maximum that a driver should divert from the basic driving task. A two-way stop requires a driver to cross traffic streams from either direction; this poses a potential risk, because cross traffic may be proceeding rapidly and drivers may be less prepared to accommodate to errors made by crossing or turning drivers. As noted by Harwood et al. Of course, a conflict must be seen before any cognitive processing of this sort proceeds. The 15th percentile speeds were 4.09 ft/s and 3.19 ft/s for younger and older pedestrians, respectively. This suggests that at stop-controlled intersections, older drivers in particular, older female driversare more likely to collide with speeding vehicles approaching on the major road. It therefore deserves emphasis that the goal of increased response times for aging drivers, requiring longer visibility distances, can also be provided by ensuring that the available signal strength (peak intensity) is maintained through a wide, versus a narrow, viewing angle. Drivers age 66+ had low correct response rates (29%) for the permissive circular green ball when shown with the red through indication. The other factor involves the ability to turn the steering wheel sharply enough, given the speed at which they are traveling, to remain within the boundaries of their lanes. The sample included 101 intersections with at least one intersecting roadway with a speed limit of 45 mph or higher. Although there is no consensus from the above studies on the actual values of PRT that should be employed in the ISD computations, there is a very clear concern as to whether the current values are meeting the needs of aging drivers. Fisher and Cole (1974), using data from Blackwell (1970), suggested that aging drivers may require 1.5 times the intensity at 50 years of age and 3 times the intensity at 70 years of age, and protanopes (individuals with a color-vision deficiency resulting in partial or full insensitivity to red light) may require a fourfold increase. This results from a decline in contrast sensitivity and depth perception. The Gap Acceptance model developed and proposed to replace the 1994 AASHTO ISD model is: Field data were collected in the NCHRP study to better quantify the gap acceptance behavior of passenger car and truck drivers, but only for left- and right-turning maneuvers from minor roadways controlled by a STOP sign (Cases IIIB and C). If the signals and markings are not understood, at a minimum there may be delay in making a turn or, in the worst case, a crash could result if a protected operation is assumed where it does not exist. Even with an RTOR prohibition, approximately 20 percent of motorists committed an RTOR violation when given the opportunity (Zegeer and Cynecki, 1986). Ruddock, 1965); this suggests a potential benefit to aging drivers of the "yellow tint" of high-pressure sodium highway lighting installations. Encroaching upon a far lane can lead to conflict with vehicles approaching from the right or, on multilane roads, oncoming drivers turning to their left at the same time. The authors noted that the smaller safety effects for the group of urban multilane roundabouts suggests that there may be differences in safety performance for single-lane designs compared to multi-lane designs. Testing was conducted using a controlled field test facility, where subjects drove their own vehicles. Standardize the sequence for the left-turn green arrow so that it precedes solid green or red. At approaches having three opposing lanes of traffic, the statistics for conversions from leading protected-only to leading protected/permissive and vice versa reinforced each other, suggesting that leading protected-only is safer than leading protected/permissive. They offered that the circular red symbol on the sign helps draw drivers' attention to it, particularly as intersections are associated with a preponderance of signs and information, and recommended that it should be added to theMUTCDas an alternate or approved as a replacement to the current R10-11a design. The need for appropriate levels of brightness to ensure conspicuity and timely detection by drivers of highway signs, including STOP and YIELD signs, was addressed in FHWA-sponsored research to establish minimum retroreflectivity requirements for these devices (minimum maintained levels, as opposed to new or in-service levels). vertical curves to satisfy the comfort criteria over the typical design vertical curves (Figure 17), and sight distance at undercrossings (Figure Based on intersections examined in the study, the offset necessary to achieve unrestricted sight distance for opposing left-turning cars is 4.1 ft and for opposing left-turning trucks is 5.6 ft. The point is that from an operational perspective, hesitancy as a result of misunderstanding will decrease the level of service and possibly result in crash situations. They also serve as a safety zone for pedestrians. AASHTO (2011) presents the principles for modern roundabouts and discusses the need to accommodate all modes; theGreen Bookprovides some degree of specific design guidance in Section 9.10, but it refers the reader to NCHRP Report 672, which is the second edition of FHWA'sRoundabouts: An Informational Guide(FHWA, 2010) and provides more detail on specific design parameters. When the crash statistics were stratified by various ranges of left-turn volume and various ranges of opposing volume (vehicles per day), the following observations and conclusions were made for sample sizes greater than five, eliminating any conclusions about lagging protected-only phasing: Leading protected-only phasing had the lowest left-turn crash rate in almost every case. Researchers examining the State crash records of 53 aging drivers found that those with restrictions in their "useful field of view," a measure of selective attention and speed of visual processing, had 15 times more intersection crashes than those with normal visual attention (Owsley, et al., 1991). The sample contained 88 drivers age 65 and older. less. In addition, the percentage of drivers with less than 30 degrees of horizontal peripheral vision increased with increases in age, from 15 percent of the younger driver sample to 65 percent of the drivers age 70+. There was also a significant reduction in the percentage of pedestrians who were running or had an aborted crossing after the countdown signal installation. Description of Practice:Within the last five years, interest in a set of intersection designs collectively called "innovative" or "alternative" has grown rapidly. Absolute minimum widths of 9 ft should be used only in unusual circumstances, and only on low-speed streets with minor truck volumes.". This is the case with street name signs at intersections. One hundred subjects divided across three age groups drove their own vehicles around test routes using the local street network in Arlington, VA. It is generally agreed that the visibility issues associated with circular signals relate to the following factors: minimum daytime intensity, intensity distribution, size, nighttime intensity, color of signals, backplates, depreciation (light loss due to lamp wear and dirt on lenses), and phantom (apparent illumination of a signal in a facing sun). restrictions and where they occur. The crash rate at the 21 study intersections decreased by 51 percent for total crashes and by 38 percent for ran-STOP-sign crashes. Thus, for a sidewalk whose centerline is 6 ft from the roadway edge, a 15-ft corner radius increases the crossing distance by only 3 ft. Older road users do not necessarily react more slowly to events that are expected, but they take significantly longer to make decisions about the appropriate response than younger road users, and this difference becomes more exaggerated in complex situations. Pupil size is reduced among aging individuals through senile miosis (Owsley, 1987), which is most detrimental at night because the reduction in light entering the eye compounds the problem of light lost via the ocular media, as described above. The signs were placed on the far side of the intersection, either on the right or the left side, and the drivers' task was to read aloud the street name as soon as it was legible to them, as they approached at a speed of 20 mph. They also noted that legibility losses with age are greater at low levels of background luminance. The rationale for treatments pertaining to intersection sight distance requirements will proceed as follows. WebAmerican Association of Highway and Transportation Officials (AASHTO) is a nonprofit association that represents highway and transportation departments across the nation The procedures used in the design of curb radii are well detailed in theGreen Book(AASHTO, 2011). The increased mobility exhibited by the two younger groups of drivers at the channelized right-turn lane locations was not, however, exhibited by the drivers age 75 and older, who stopped in 19 of the 20 turns executed at the channelized locations. Lighting provides a particular benefit to aging drivers by increasing expectancy of needed vehicle control actions, at longer preview distances. The authors explain that uppercase words look like blurry rectangles when viewed from a distance. Book provides design criteria for decision sight distance, passing The results of the Harwood et al. Similarly, the reduction in decision time that should be realized from centering the signal over the approach lane will be of greatest benefit to aging drivers with reduced speed of processing who face the highest demand for "executive control" when negotiating an intersection.

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