New Headway Intermediate Fourth Edition Stop And Check Unit 5 8
Bernd Manison
There is broad agreement that right-angle intersections are the preferred design. Decreasing the angle of the intersection makes detection of and judgments about potential conflicting vehicles on crossing roadways much more difficult. In addition, the amount of time required to maneuver through the intersection increases, for both vehicles and pedestrians, due to the increased pavement area. However, there is some inconsistency among reference sources concerning the degree of skew that can be safely designed into an intersection. The Green Book states that although a right-angle crossing normally is desired, an angle of 60 degrees provides most of the benefits that are obtained with a right-angle intersection. Subsequently, factors to adjust intersection sight distances for skewness are suggested for use only when angles are less than 60 degrees (AASHTO, 2011). However, another source on subdivision street design states that: "Skewed intersections should be avoided, and in no case should the angle be less than 75 degrees" (Institute of Transportation Engineers [TEH], 1984). The Traffic Engineering Handbook (TEH, 1999) states that: "Crossing roadways should intersect at 90 degrees if possible, and not less than 75 degrees." It further states that: "Intersections with severe skew angles (e.g., 60 degrees or less) often experience operational or safety problems. Reconstruction of such locations or institution of more positive traffic control such as signalization is often necessary." With regard to intersection design issues on two-lane rural highways, TEH (1999) states that: "Skew angles in excess of 75 degrees often create special problems at stop-controlled rural intersections. The angle complicates the vision triangle for the stopped vehicle; increases the time to cross the through road; and results in a larger, more potentially confusing intersection."
new headway intermediate fourth edition stop and check unit 5 8
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Skewed intersections pose particular problems for aging drivers. Many aging drivers experience a decline in head and neck mobility, which accompanies advancing age and may contribute to the slowing of psychomotor responses. Joint flexibility, an essential component of driving skill, has been estimated to decline by approximately 25 percent in aging adults due to arthritis, calcification of cartilage, and joint deterioration (Smith and Sethi, 1975). A restricted range of motion reduces an aging driver's ability to effectively scan to the rear and sides of his or her vehicle to observe blind spots, and similarly may be expected to hinder the timely recognition of conflicts during turning and merging maneuvers at intersections (Ostrow, Shaffron, and McPherson, 1992). For aging drivers, diminished physical capabilities may affect their performance at intersections designed with acute angles by requiring them to turn their heads further than would be required at a right-angle intersection. This obviously creates more of a problem in determining appropriate gaps. For aging pedestrians, the longer exposure time within the intersection becomes a major concern.
Isler, Parsonson, and Hansson (1997) measured the maximum head rotation of 20 drivers in each of four age groups: less than age 30; ages 40 to 59; ages 60 to 69; and age 70 and older, as well as their horizontal peripheral visual field. The oldest subjects exhibited an average decrement of approximately one-third of head range of movement compared with the youngest group of subjects. The mean maximum head movement (in one direction) was 86 degrees for the youngest drivers, 72 degrees for drivers ages 40 to 59, 67 degrees for drivers ages 60 to 69, and 59 degrees for drivers age 70+. 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+. Three of the oldest drivers had less than 50 degrees of head movement and two of these drivers also had less than 20 degrees of horizontal peripheral vision.
In a survey of aging drivers conducted by Yee (1985), 35 percent of the respondents reported problems with arthritis and 21 percent indicated difficulty in turning their heads to scan rearward while driving. Excluding vision/visibility problems associated with nighttime operations, difficulty with head turning placed first among all concerns mentioned by aging drivers participating in a focus group conducted by Staplin, Harkey, Lococo, and Tarawneh (1997) to examine problems in the use of intersections where the approach leg meets the main road at a skewed angle, and/or where channelized right-turn lanes require an exaggerated degree of head/neck rotation to check for traffic conflicts before merging. Comments about this geometry centered around the difficulty aging drivers experience turning their heads at angles less than 90 degrees to view traffic on the intersecting roadway, and several participants reported an increasing reliance on outside rearview mirrors when negotiating highly skewed angles. However, they reported that the outside mirror is of no help when the roads meet at the middle angles (e.g., 40 to 55 degrees) and a driver is not flexible enough to physically turn to look for traffic.
Older and younger driver performance was compared at 10 intersections (5 improved and 5 unimproved) to test the effectiveness of FHWA's recommendations for intersection design to accommodate aging road users (Classen et al., 2007). Thirty-nine drivers ages 25 to 45 and 32 drivers ages 65 to 85 drove an instrumented vehicle on urban and residential streets in Gainesville, FL, accompanied by a front-seat driving evaluator who recorded behavioral errors. The course took approximately 1 hour to complete, and included driving through 5 sets of improved and unimproved intersections. One set of intersections included roadways that met at a 90-degree angle (improved) and roadways that met at an angle less than 75 degrees (unimproved). Both kinematic data (vehicle control responses during the turn phase including longitudinal and lateral accelerations, yaw, and speed) and behavioral data (driving errors including vehicle position, lane maintenance, speed, yielding, signaling, visual scanning, adjustment to stimuli/traffic signs, and left-turn gap acceptance) were recorded. With the exception of speed during the turn, kinematic measures showed significantly better performance associated with the improved intersection, and there were significantly fewer behavioral errors with the improved design. The improved design was associated with lesser side forces, indicating improved lateral stability, and fewer deviations from the idealized curved path during the turn, indicating greater vehicle stability. There were no significant differences between age groups for either the kinematics measures or the behavioral measure.
These research findings reinforce the desirability of providing a 90-degree intersection geometry and support the TEH (1984) recommendation establishing a 75-degree minimum as a practice to accommodate age-related performance deficits, benefiting both older as well as younger drivers.
Lane widths are addressed in the Intersection Channelization Design Guide (Neuman, 1985). A recommendation for (left) turning lanes, which also applies to receiving lanes, is that "12-ft widths are desirable, (although) lesser widths may function effectively and safely. Absolute minimum widths of 9 ft should be used only in unusual circumstances, and only on low-speed streets with minor truck volumes." Similarly, the TEH (1984) guidelines suggest a minimum lane width of 11 ft and specify 12 ft as desirable. These guidelines suggest that wider lanes be avoided due to the resulting increase in pedestrian crossing distances. However, the TEH guidelines provide a range of lane widths at intersections from 9 ft to 14 ft, where the wider lanes would be used to accommodate larger turning vehicles, which have turning paths that sweep a path from 13.6 ft for a single-unit truck or bus up to 20.6 ft for a semitrailer. Thus, wider (12-ft) lanes used to accommodate (right) turning trucks also are expected to benefit (left) turning drivers. Further increases in lane width for accommodation of heavy vehicles may result in unacceptable increases in (aging) pedestrian crossing times, however.
Results of field observation studies conducted by Firestine, Hughes, and Natelson (1989) found that trucks turning on urban roads encroached into other lanes on streets with widths of less than 12 ft. They noted that on rural roads, lanes wider than 12 ft or 13 ft allowed oncoming vehicles on the cross street to move further right to avoid trucks, and shoulders wider than 4 ft allowed oncoming vehicles a greater margin of safety. Lane widths are addressed in the Intersection Channelization Design Guide (Neuman, 1985). A recommendation for (left) turning lanes, which also applies to receiving lanes, is that "12-ft widths are desirable, (although) lesser widths may function effectively and safely. Absolute minimum widths of 9 ft should be used only in unusual circumstances, and only on low-speed streets with minor truck volumes."
Design recommendations for lane width at intersections follow from consideration of vehicle maneuver requirements and their demands on drivers. Positioning a vehicle within the lane in preparation for turning has long been recognized as a critical task (McKnight and Adams, 1970). Swinging too wide to lengthen the turning radius and minimize rotation of the steering wheel ("buttonhook turn") while turning left or right is a common practice of drivers who lack strength (including aging drivers) or are physically limited (McKnight and Stewart, 1990).
Two factors can compromise the ability of aging drivers to remain within the boundaries of their assigned lane during a left turn. One factor is the diminishing ability to share attention (i.e., to assimilate and concurrently process multiple sources of information from the driving environment). 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. Some aging drivers seek to increase their turning radii by initiating the turn early and rounding-off the turn. The result is either to cut across the apex of the turn, conflicting with vehicles approaching from the left, or to intrude upon a far lane in completing the turn.
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