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Human Factors Guidelines for Road Systems: Second Edition (2012)

Chapter: Chapter 10 - Non-Signalized Intersections

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Suggested Citation:"Chapter 10 - Non-Signalized Intersections." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 10 - Non-Signalized Intersections." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 10 - Non-Signalized Intersections." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 10 - Non-Signalized Intersections." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 10 - Non-Signalized Intersections." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 10 - Non-Signalized Intersections." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 10 - Non-Signalized Intersections." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 10 - Non-Signalized Intersections." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 10 - Non-Signalized Intersections." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 10 - Non-Signalized Intersections." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 10 - Non-Signalized Intersections." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Acceptable Gap Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2 Factors Affecting Acceptable Gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-4 Sight Distance at Left-Skewed Intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-6 Sight Distance at Right-Skewed Intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-8 Countermeasures for Improving Accessibility for Vision-Impaired Pedestrians at Roundabouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-10 10-1 C H A P T E R 10 Non-Signalized Intersections

ACCEPTABLE GAP DISTANCE Introduction Acceptable gap distance refers to the size of the gaps in major-road traffic typically accepted by drivers turning from a minor road that provide sufficient time for the minor-road vehicle to accelerate from stop and complete a turn without unduly interfering with major-road traffic operations. A constant-value of time gap, independent of approach speed, can be used for determining intersection sight distance (see AASHTO (1)). In particular, the intersection sight distance in both directions should be equal to the distance traveled at the design speed of the major road during a period of time equal to the gap. Design Guidelines Time Gap (tg) at Design Speed of Major Road Design Vehicle Left Turn Right Turn Passenger Car 7.5 s 6.5 s Single-Unit Truck 9.5 s 8.5 s Combination Truck 11.5 s 10.5 s Note: Time gaps are for a stopped vehicle to turn onto a two-lane highway with no median and grades of 3% or less. The table values require adjustment as follows: For multilane highways: For turns onto highways with more than two lanes, add 0.5 s for passenger cars or 0.7 s for trucks for each additional lane (including narrow medians that cannot store the design vehicle), in excess of one, to be crossed by the turning vehicle. For left turn onto minor roads with approach grades: If the approach grade is an upgrade that exceeds 3%, add 0.2 s for each percent grade. For right turn on minor roads with approach grades: If the approach grade is an upgrade that exceeds 3%, add 0.1 s for each percent grade. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data The figure below shows the different aspects of gap acceptance in a left-turn situation. Gaps are defined as the time interval between two successive vehicles, measured from the rear of a lead vehicle to the front of the following vehicle. Lags are defined as the time interval from the point of the observer to the arrival of the front of the next approaching vehicle. To account for the time needed to traverse additional lanes add 0.5 s for passenger cars and 0.7 s for trucks Vehicle approaching from the left Vehicle approaching from the right HFG NON-SIGNALIZED INTERSECTIONS Version 2.0 10-2

Discussion Safe gap acceptance distances depend on the driver’s ability to accurately judge the time available to execute a traffic- crossing maneuver. Chovan, Tijerina, Everson, Pierowicz, and Hendricks (2) indicate that failure to accurately perceive and judge safe gap distances can have serious safety consequences, and that two of the most common causal factors for left turn crashes are misjudged gap/velocity (30% to 36% of crashes) and drivers misperceiving (e.g., “looked but did not see”) oncoming traffic (23% to 26% of crashes). At short distances, where the size of the visual image (on the observer’s retina) of the oncoming traffic is relatively large, time-to-arrival judgments may be made based on optical properties of the scene, such as the observed rapid expansion (“looming”) of the visual image as the object approaches (see, for example, Kiefer, Cassar, Flannagan, Jerome, and Palmer (3)). However, at the distances involved in roadway gap judgments there is less agreement about whether these optical properties are as important or if other aspects, such as speed and distance judgments, dominate. In general, however, observers are not particularly adept at making judgments about arrival time and they tend to underestimate this value by 20% to 40% (4). Fortunately, the degree of underestimation is reduced with higher oncoming vehicle speed and with longer viewing duration (4). One study found that distance from oncoming vehicle was the best predictor of gap acceptance, while vehicle speed and time-to-arrival were weaker predictors (5). This finding suggests that drivers are somewhat insensitive to oncoming vehicle speed, which means that they may be more likely to accept smaller/less-safe gaps if the speeds of oncoming vehicles are higher. Also, this effect appears to be more pronounced in older drivers than in younger drivers (6). The data for the acceptable gap distance guideline come from Harwood, Mason, and Brydia (7), which measured critical gap for use as an intersection sight distance criterion. For design purposes, the critical gap represents the gap between successive oncoming vehicles that average drivers will accept 50% of the time (and reject 50% of the time). The rationale for using critical gap as an ISD criterion is that if drivers will accept a specific critical gap in the major-road traffic stream when making a turning maneuver, then sufficient ISD should be provided to enable drivers to identify that critical gap. The key findings from Harwood et al. (7), which are reflected in the guideline, are that drivers accept slightly shorter gaps for right turns than for left turns, and that heavy vehicles require longer gaps. Note, however, that other studies have not found a difference in gap acceptance size based on turn direction. In particular, one study found that passenger vehicle drivers accepted a critical gap of 6.5 s for both left and right turns; this source also reviewed comparable studies that also found mixed results regarding the effect of turn direction (8). Another factor that must be considered is the direction from which drivers face conflicting traffic. In particular, Kittelson and Vandehey (9) found that left-turning drivers will accept shorter gaps if the gap they are evaluating involves a vehicle approaching from the left rather than from the right. Design Issues Vehicles approaching the turning/crossing vehicle can be expected to slow down to avoid any potential conflicts; however, this deceleration may impact capacity on high-volume roadways. Harwood et al. (7) found that for turns executed with gaps of less than 10 s, oncoming vehicles decelerated from 0% to 80% with a median deceleration of 31% (average deceleration level was 0.68 m/s2). On average, two-thirds of the speed reduction occurs before the oncoming vehicle reaches the intersection. The average acceleration level of the turning vehicle was 1.46 m/s2. Cross References Determining Intersection Sight Distance, 5-6 Factors Affecting Acceptable Gap, 10-4 Key References 1. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington DC. 2. Chovan, J., Tijerina, L., Everson, J., Pierowicz, J., and Hendricks, D. (1994). Examination of Intersection, Left Turn Across Path Crashes and Potential IVHS Countermeasures (DOT HS 808 154). Washington, DC: National Highway Traffic Safety Administration. 3. Kiefer, R.J., Cassar, M.T., Flannagan, C.A., Jerome, C.J., and Palmer, M.D. (2005). Surprise Braking Trials, Time-to-Collision Judgments, and “First Look” Maneuvers under Realistic Rear-End Crash Scenarios (Forward Collision Warning Requirements Project, Tasks 2 and 3a Final Report, DOT HS 809 902). Washington, DC: National Highway Traffic Safety Administration, Office of Advanced Safety Research. 4. Groeger, J.A. (2000). Understanding Driving: Applying Cognitive Psychology to a Complex Everyday Task. Hove, U.K.: Psychology Press. 5. Davis, G.A., and Swenson, T. (2004). Field study of gap acceptance by left-turning drivers. Transportation Research Record, 1899, 71-75. 6. Staplin, L. (1995). Simulator and field measures of driver age differences in left-turn gap judgments. Transportation Research Record, 1485, 49-55. 7. Harwood, D.W., Mason, J.M., Jr., and Brydia, R.E. (2000). Sight distance for stop-controlled intersections based on gap acceptance. Transportation Research Record, 1701, 32-41. 8. Fitzpatrick, K. (1991). Gaps accepted at stop-controlled intersections. Transportation Research Record, 1303, 103-112. 9. Kittelson, W.K., and Vandehey, M.A. (1991). Delay effects on driver gap acceptance characteristics at two-way stop-controlled intersections. Transportation Research Record, 1320, 154-159. 10-3 HFG NON-SIGNALIZED INTERSECTIONS Version 2.0

FACTORS AFFECTING ACCEPTABLE GAP Introduction The factors affecting acceptable gap refer to the driver, environment, and other situational factors that cause most drivers or specific groups of drivers (e.g., older drivers) to accept smaller or larger gaps than they would otherwise accept under normal conditions. These guidelines only apply when there is no center median or acceleration lane that provides shelter to the turning vehicle. Design Guidelines Certain driver, environmental, or situational factors can systematically influence driver gap acceptance behavior. If these factors are common at an intersection location, then consideration should be given to modifying the gap acceptance design assumptions. Factor Finding Data Quality* Driver Age Older drivers accept a critical gap that is approximately 1 s longer than younger drivers, and they reject more acceptable gaps overall. Wait Times Critical gap size decreases as a function of time spent waiting at the stop line. Direction of Turn Drivers will accept shorter gaps if the primary conflicting vehicle is approaching from the driver’s left than if it is from the driver’s right (same destination lane). Familiarity with Roadway Drivers on familiar routes (e.g., work commutes) accept smaller critical gaps. Oncoming Vehicle Size Larger vehicles are perceived as arriving sooner than smaller vehicles. Traffic Volume Drivers accept smaller gaps with higher major-road traffic volume. Headlight Glare Drivers accept longer critical gaps with oncoming headlight glare. *Data Quality: = established finding; = some empirical evidence, but magnitude of effect and reliability of findings are unconfirmed. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data The table below shows the perceptual, cognitive, and psychomotor subtasks associated with the key activities that drivers must perform when making left or right turns across traffic in a four-lane roadway (adapted from Richard, Campbell, and Brown (9)). Activity Perceptual Subtasks Cognitive Subtasks Psychomotor Subtasks 1. Check for possible conflicts with following vehicle. Visually assess trajectory of following vehicle. Determine if distance and speed of vehicle indicate potential conflict. Head and eye movements to observe rearview mirror. 2. Check for pedestrians/cyclists crossing or about to cross in front. Look left and right along crosswalk. Determine if pedestrians/cyclists are present or likely to enter the crosswalk. Head and eye movements for viewing. 3. Advance into crosswalk. Visually observe crosswalk. Determine when vehicle is in appropriate position for turning. Slowly accelerate and brake. 4. Look for gap in perpendicular traffic. Visually monitor traffic. Determine distance and speed of oncoming traffic. Determine if gap is sufficient for turning. Head and eye movements to monitor oncoming traffic. 5. Check for oncoming vehicles in far lane changing to destination (conflicting) lane. Monitor oncoming vehicles in far lane. Determine if vehicle is about to change lanes (e.g., turn signal on, changing trajectory, etc.). Head and eye movements to monitor oncoming traffic. 6. Check for hazards in turn path. Visually scan turn path (especially crosswalk) and intended lane. Determine if any pedestrians/cyclists or other hazards are in the crosswalk or about to enter. Head and eye movements to view turn path. 7. Accelerate to initiate turn. View roadway. Determine that acceleration is sufficient to avoid conflicts with other vehicles Quickly accelerate. Head and eye movements. 8. Steer into turn. View turn path. Determine that vehicle trajectory and lane position are appropriate. Steering adjustments necessary to stay in lane. HFG NON-SIGNALIZED INTERSECTIONS Version 2.0 10-4

Discussion Driver age: Several studies have found that older drivers require gaps that are approximately 1 s longer than younger drivers. Some studies also find that older drivers tend to reject more usable gaps than other drivers, which leads to capacity reductions (1, 2). The data suggest that these differences likely reflect more cautious decision criteria (1). Yi (2) also found that older drivers require more time to enter and accelerate to the desired speed (10–13 s to reach 25 mi/h and 16–19 s to reach 35 mi/h compared to the respective 7–9 s and 12–14 s for younger drivers). Wait times: Most vehicles that wait in a queue accept smaller gaps than those that do not wait (3). Also, the longer that drivers wait, the more likely they are to accept gaps that they previously rejected as being too short (4). Note that there is no information about whether this arises from increased driver frustration or from drivers learning through observation that smaller gaps are likely to be safe (3). Direction of turn: Drivers accept shorter gaps if the primary other vehicle is approaching from the driver’s left than if it is approaching from the driver’s right (4, 5). For example, a driver making a left turn will accept a smaller gap from a vehicle approaching from the left (for which there will only be a conflict while the turning vehicle crosses its path), than one approaching from the right (for which there will be a potential conflict until the turning vehicle gets up to speed). If drivers are faced with a single vehicle coming in the conflicting direction, then some data suggest that drivers will accept shorter gaps while making right turns than left turns (6); however, there is also evidence that this difference is small or insignificant. Familiarity with the roadway: Only one study considered the effects of driver familiarity on gap acceptance (5). This study found that drivers on regular commute trips generally accept smaller gaps, which seems to arise because drivers are familiar with what constitutes a safe gap in a particular turn situation. Oncoming vehicle size: Some driving simulator research indicates that larger vehicles are perceived as arriving sooner than smaller vehicles, even if their actual arrival time is the same (7). This finding may have implications for roadways with high motorcycle traffic, because drivers may overestimate the gap size for these smaller vehicles. Traffic volume: Higher traffic volume on the major road appears to lead to drivers accepting smaller gaps (3). This situation could arise because large gaps are less common or drivers see the need to take whatever gap is available, even if it is smaller than what they would normally take. Headlamp glare: Data from a study involving unlit rural conditions indicated that accepted gaps were significantly larger under higher glare conditions from approaching vehicles, although the lighting systems used were from the late 1960s and therefore the data may be less applicable today (8). Design Issues None. Cross References Determining Intersection Sight Distance, 5-6 Acceptable Gap Distance, 10-2 Key References 1. Lerner, N., Huey, R.W., McGee, H.W., and Sullivan, A. (1995). Older Driver Perception-Reaction Time for Intersection Sight Distance and Object Detection. Volume I, Final Report. (FHWA-RD-93-168). Washington, DC: FHWA. 2. Yi, P. (1996). Gap acceptance for elderly drivers on rural highways. Compendium of Technical Papers for the 66th ITE Annual Meeting (pp. 299- 303). Washington, DC: ITE. 3. Kyte, M., Clemow, C., Mahfood, N., Lall, B.K., and Khisty, C.J. (1991). Capacity and delay characteristics of two-way stop-controlled intersections. Transportation Research Record, 1320, 160-167. 4. Kittelson, W.K., and Vandehey, M.A. (1991). Delay effects on driver gap acceptance characteristics at two-way stop-controlled intersections. Transportation Research Record, 1320, 154-159. 5. Hamed, M.M., Easa, S.M., and Batayneh, R.R. (1997). Disaggregate gap-acceptance model for unsignalized T-intersections. Journal of Transportation Engineering, 123(1), 36-42. 6. Harwood, D.W., Mason, J.M., and Brydia, R.E. (2000). Sight distance for stop-controlled intersections based on gap acceptance. Transportation Research Record, 1701, 32-41. 7. Caird, J., and Hancock, P. (1994). The perception of arrival time for different oncoming vehicles arriving at an intersection. Ecological Psychology, 6, 83-109. 8. Tsongos, N.G., and Schwab, R.N. (1970). Driver judgments as influenced by vehicular lighting at intersections. Highway Research Record, 336, 21-32. 9. Richard, C.M., Campbell, J.L., and Brown, J.L. (2006). Task Analysis of Intersection Driving Scenarios: Information Processing Bottlenecks (FHWA-HRT-06-033). Washington, DC: FHWA. 10-5 HFG NON-SIGNALIZED INTERSECTIONS Version 2.0

SIGHT DISTANCE AT LEFT-SKEWED INTERSECTIONS Introduction Sight distance at left-skewed intersections refers to the available sight distance to the driver’s right side for a vehicle crossing a major road from a left-skewed minor road (where the acute angle is to the right of the vehicle). In left- skewed intersections, the driver’s line of sight can be obstructed by parts of the driver’s vehicle, such as the roof posts, door frame, passenger-seat headrest, or a panel aft of the door. This obstruction is most likely to occur for vehicles that have vision-restricting rearward elements, for example, ambulances, motor homes, truck tractors with sleeping areas, single-unit trucks, and school buses. These sight-line restrictions can result in reduced sight distances because the driver cannot see as far down the intersecting road as with 90° intersections. AASHTO (1) recommends that intersection angles be skewed no more than 60°; however, as Gattis and Low (2) indicate, intersections that are skewed from 60° to 70° can still result in insufficient sight distance for vision-restricted vehicles at certain design speeds. The guideline provides information about available sight distance (ASD) and the design speed that accommodates the ASD for different viewing/vision angles. Two different vision angle conditions are presented. The minimum vision angle indicates design parameters for the minimum recommended vision angle. The desirable vision angle provides more conservative recommended values that better accommodate larger vehicles and older drivers. Design Guidelines Design speeds for the major roadway should be consistent with available sight distance for the minor-road vehicle based on at least the minimum vision angle viewing position, but use of the desirable vision angle is preferable and better accommodates larger vehicles and older drivers. RESULTING AVAILABLE SIGHT DISTANCE FOR 5.4-M AND 4.4-M SETBACKS* Resulting ASD for a 5.4-m Setback Resulting ASD for a 4.4-m Setback Minimum Vision Angle: 13.5° Desirable Vision Angle: 4.5° Minimum Vision Angle: 13.5° Desirable Vision Angle: 4.5° Intersection Angle (degrees) ASD (m) Design Speed (km/h) ASD (m) Design Speed (km/h) ASD (m) Design Speed (km/h) ASD (m) Design Speed (km/h) 55 31.8 31 23.6 <30 – – – – 60 39.8 37 26.9 <30 36.4 35 24.6 <30 65 55.4 46 32.3 32 50.5 43 29.5 30 70 95.7 65 41.6 38 87.1 61 37.8 36 75 408.2 >120 60.1 49 371.1 >120 54.6 46 *Calculations assume a W (see figure below) of 5.4 based on 1½ lane widths of 3.6 m. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data The figure below shows the variables and dimensions used to calculate the ASD and design speed values in the table. A Subject Vehicle (SV) Collision Distance E Setback Distance B Available Sight Distance IA Intersection Skew Angle VA Subject Vehicle Vision Angle W Distance from Principal Other Vehicle (POV) to Intersection Tangent Line L Lane Width (3.6 m) W Dr ive r Ey e Po siti on L B A E IA VA HFG NON-SIGNALIZED INTERSECTIONS Version 2.0 10-6

Discussion The available sight distances presented in the guideline are calculated based on drivers of restricted-vision vehicles viewing oncoming traffic backwards over their right shoulder. The 4.5° viewing-angle condition represents a driver sitting back fully against the seat, which represents the most restricted viewing-angle condition. Actual viewing angles from this position can range from around 2° in ambulances and motor homes to 7° or 8° in single-unit trucks and school buses. Viewing angles are typically more than 19° in all vehicles if drivers adopt an extreme “leaning forward” position in which their head is positioned almost directly above the steering wheel (2). The 13.5° viewing- angle condition used in the guideline represents an intermediate “leaning forward” driver posture that is between the “fully against the seat back” position and the “full forward” position. It was selected based on expert judgment of a review panel involved in the study and represents a reasonable approximation of how far forward most drivers could be expected to lean. Son, Kim, and Lee (3) measured the available vision angle in three Korean design vehicles (passenger cars, single- unit trucks, and semi-trailers). The viewing angles in single-unit trucks and semi-trailers were 1.3° in the “seat back” position and 12.6° to 13.1° in the “full forward” position. However, viewing angles from a comfortable “leaning forward” position in these vehicles were 5.2° to 5.4°, which are smaller than the 13.5° viewing angle adopted for the guideline. Viewing angles for passenger cars were much greater, having values of 13.5° and 17° in the “seat back” and comfortable “leaning forward” positions, respectively. It should be noted that some drivers, especially older drivers, may be restricted in their ability to lean forward because of limitations in their neck and trunk flexibility, and therefore the intermediate “leaning forward” position (13.5°) may be difficult to obtain. If the design must accommodate older drivers, use of the desirable vision angle may be more appropriate. See the guideline “Sight Distance at Right-Skewed Intersections” for additional discussion of this issue. The design speed measure reported in the guideline is based on the time available for the vehicle on the major road to stop or avoid a conflict with the minor-road vehicle that entered the intersection late based on what its driver could see from the restricted viewing angle. Note that vehicles passing through skewed intersections also have a longer distance to traverse, which increases the driver’s exposure to oncoming traffic. The 5.4-m setback represents a conservative estimate for how far back the driver’s eye position is from the edge of the major road. More specifically, it is based on the distance of 5.4 m measured from the minor-road vehicle driver’s eye to the edge of the cross road. This value is the recommended driver-position setback for intersection sight distance calculations (4). However, a setback distance of 4.4 m may also be used for constrained situations and is consistent with driver behavior in response to restricted sightline situations. Design Issues To what extent the current recommendations apply to light trucks is uncertain at this point. Restricted rearward viewing may occur with light trucks because some lack a rear seating area with windows and some have truck bed attachments that can obscure the rearward view. Cross References Determining Intersection Sight Distance, 5-6 Sight Distance at Right-Skewed Intersections, 10-8 Key References 1. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington DC. 2. Gattis, J.L. and Low, S.T. (1998). Intersection angle geometry and the driver's field of view. Transportation Research Record, 1612, 10-16. 3. Son, Y.T., Kim, S.G., and Lee, J.K. (2002). Methodology to calculate sight distance available to drivers at skewed intersections. Transportation Research Record, 1796, 41-47. 4. Harwood, D.W., Mason, J.M., Brydia, R.E., Pietrucha, M.T., and Gittings, G.L. (1996). Appendix H: Field studies of vehicle dimensions and vehicle-stopping positions on minor-road approaches to stop-controlled intersections. Contractor’s Final Report, NCHRP Project 15- 14(1).Washington, DC: Transportation Research Board. 10-7 HFG NON-SIGNALIZED INTERSECTIONS Version 2.0

SIGHT DISTANCE AT RIGHT-SKEWED INTERSECTIONS Introduction Sight distance at right-skewed intersections refers to the available sight distance to the driver’s left side for a vehicle crossing a major road from a right-skewed minor road (where the acute angle is to the left of the vehicle). In right- skewed intersections, the drivers’ line of sight over their left shoulder is not typically obstructed by parts of their vehicle, such as the case with left-skewed intersections. In contrast, the primary limitations to drivers’ line of sight are their ability to physically turn their body to the left and how far over their shoulder they can orient their gaze to view oncoming vehicles. These viewing limitations can result in reduced sight distances because the driver can not see as far down the intersecting road as they could at a 90° intersection. The guideline provides recommendations for accommodating older drivers who are more likely to have neck and/or trunk movement restrictions, in addition to recommendations for drivers without such limitations (identified as “other drivers”). Design Guidelines Design speeds for the major roadway should be consistent with available sight distance (ASD) for the minor-road vehicle based on at least the vision angle for drivers without neck and/or trunk movement restrictions (other-driver); however, the use of the older-driver vision angle better accommodates older drivers and those drivers with neck and/or trunk movement restrictions regardless of age. RESULTING AVAILABLE SIGHT DISTANCE FOR 5.4-M AND 4.4-M SETBACKS* Resulting ASD for a 5.4-m Setback Resulting ASD for a 4.4-m Setback Other-Driver Vision Angle: 115° Older-Driver Vision Angle: 95° Other-Driver Vision Angle: 115° Older-Driver Vision Angle: 95° Intersection Angle (degrees) ASD (m) Design Speed (km/h) ASD (m) Design Speed (km/h) ASD (m) Design Speed (km/h) ASD (m) Design Speed (km/h) 55 39.7 35.8 15.1 17.0 35.0 32.7 13.3 15.3 60 77.8 57.4 17.6 19.2 68.4 52.7 15.5 17.3 65 Not limited† >120 21.5 22.6 Not limited† >120 18.9 20.3 70 Not limited† >120 28.2 27.8 Not limited† >120 24.7 25.1 75 Not limited† >120 41.7 37.2 Not limited† >120 36.5 33.7 *Calculations assume a W (see figure below) of 1.8 based on ½ a lane width of 3.6 m. †At higher intersection angles, driver visibility is not limited by vision angle. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data The figure below shows the variables and dimensions used to calculate the ASD and design speed values used in the guideline table. A Subject Vehicle (SV) Collision Distance E Setback Distance B Available Sight Distance IA Intersection Skew Angle VA Subject Vehicle Vision Angle W Distance from Principal Other Vehicle (POV) to Intersection Tangent Line W DriverEyePosition B IA VA E A HFG NON-SIGNALIZED INTERSECTIONS Version 2.0 10-8

Discussion The primary limiting factor for visibility with right-skewed intersections is the drivers’ direct field of view based on how far over their left shoulder they can see by turning their body, head, and eyes to the left. This visibility limitation contrasts with left-skewed intersections, in which parts of the vehicle body can obstruct the drivers’ view over their right shoulder regardless of how far they can see to the side. Difficulty with head turning was one of the most frequently mentioned concerns in older-driver focus groups, and these drivers reported experiencing difficulty turning their heads at angles less than 90° to view traffic on intersecting roadways. Moreover, joint flexibility declines by an estimated 25% in older drivers because of arthritis, calcification of cartilage, and joint deterioration (1). If roadway designers need to consider older- driver capabilities in the design of skewed intersections, then use of the older-driver vision angle values from the guideline is recommended. The values in the guideline table provide estimated ASD and recommended design speed for oncoming vehicles based on approximations of how far to the left drivers on the minor road can be expected to see. The ASD and design speed values in the guideline table were computed using an analogous approach to the one taken in the guideline “Sight Distance at Left-Skewed Intersections,” which is based on Gattis and Low (5). Specifically, these terms represent the time available for a vehicle on the major road to stop or avoid a conflict with the minor-road vehicle that entered the intersection based on what its driver could see from the restricted viewing angle. The minor-road driver’s viewing angle is calculated using estimated trunk, head, and eye movement capabilities observed in healthy young and middle-aged drivers (other-driver vision angle) and healthy older drivers (older-driver vision angle). For the other-driver vision angle, trunk, neck, and eye movement values of 30°, 70°, and 15° (totaling 115°) were used. For the older-driver vision angle, trunk, neck, and eye movement values of 25°, 55°, and 15° (totaling 95°) were used. No data are currently available on trunk rotation range for seated drivers restrained by safety belts. The trunk rotation value used in the guideline calculations was based on an estimate of comfortable trunk rotation range for a restrained non- older driver of 30°, and then reduced by 5° to represent reduced flexibility in older drivers. The neck rotation values are based on the study by Isler, Parsonson, and Hansson (2), which measured neck rotation to the left in seated drivers. In this study, 80% of drivers aged 59 years or younger had a neck movement range of 70° or more, while 75% of drivers aged 60 or older had a neck movement range of 55° or more. Note that these values are greater than those reported in another more comprehensive study of neck rotation, which found mean neck rotation to the left to be 65° in healthy people aged 20 to 59, and 54° in healthy people aged 60 to 79 (3). The guideline table also assumes that drivers are able to execute at least one eye movement 15° toward the left. There are no data indicating how far drivers will move their eyes when making judgments about oncoming vehicle approaches; however, most naturally occurring eye movements (saccades) have an amplitude of 15° or less, and eye movements longer than this are effortful (4). While this 15° value may be considered as representing a conservative eye movement amplitude, many older drivers have limited peripheral vision, which would make it difficult to efficiently move their eyes farther out than 15° (2). Design Issues The estimates of how far drivers can see to their left contain some degree of uncertainty, because of the lack of reliable information on driver trunk rotation and eye movement amplitude. Cross References Determining Intersection Sight Distance, 5-6 Sight Distance at Left-Skewed Intersections, 10-6 Key References 1. Staplin, L., Lococo, K., Byington, S., and Harkey, D. (2001). Guidelines and Recommendations to Accommodate Older Drivers and Pedestrians.a (FHWA-RD-01-051). McLean, VA: FHWA, Research, Office of Safety R&D. 2. Isler, R.B., Parsonson, B.S., and Hansson, G.J. (1997) Age related effects of restricted head movements on the useful field of view of drivers. Accident Analysis and Prevention, 29(6), 793-801. 3. Youdas, J.W., Garrett, T.R., Suman, V.J., Bogard C.L., Hallman H.O., and Carey J.R. (1992). Normal range of motion of the cervical spine: An initial goniometric study. Physical Therapy, 72(11), 770-80. 4. Bahill, A.T., Adler, D., and Stark, L. (1975). Most naturally occurring human saccades have magnitudes of 15 degrees or less. Investigative Ophthalmology, 14(6), 468-469. 5. Gattis, J.L. and Low, S.T. (1998). Intersection Angle Geometry and the Driver's Field of View. Transportation Research Record, 1612, 10-16. 10-9 HFG NON-SIGNALIZED INTERSECTIONS Version 2.0

COUNTERMEASURES FOR IMPROVING ACCESSIBILITY FOR VISION-IMPAIRED PEDESTRIANS AT ROUNDABOUTS Introduction This guideline identifies countermeasures for improving accessibility for vision-impaired pedestrians at roundabouts. Title II of the Americans with Disabilities Act (ADA) requires that new and altered facilities constructed by, on behalf of, or for the use of state and local government entities be designed to be readily accessible to and usable by people with disabilities (28 CFR 35.151). Also, FHWA states that “a visually impaired pedestrian with good travel skills must be able to arrive at an unfamiliar intersection and cross it with pre-existing skills and without special, intersection-specific training” (1). Roundabouts can be particularly challenging to navigate for vision-impaired pedestrians. In particular, vision-impaired pedestrians typically wait much longer to cross at roundabouts than sighted pedestrians, especially if traffic volume is high. One reason that sighted pedestrians have shorter wait times is that they can accept gaps that are initially too short but can be extended by driver yields and they can use eye gazes and manual gestures to communicate with drivers and get confirmation of driver yielding. Because vision-impaired pedestrians cannot communicate in this manner, they are forced to wait for what they deem to be sufficient gaps based on sound information. Another problem is that vision-impaired pedestrians rely heavily on sound cues to get a sense of what vehicles are doing. The continuous traffic flow within the circle can eliminate important sound cues about vehicle movements, in addition to masking sound cues from vehicles approaching the roundabout. Anecdotally, vision-impaired pedestrians typically state they would most likely avoid roundabouts if getting sufficient information about vehicle movements is too difficult. Design Guidelines COUNTERMEASURES FOR IMPROVINGACCESSIBILITY FOR VISION-IMPAIRED PEDESTRIANS AT ROUNDABOUTS Countermeasure Applicable Situation Effectiveness Rumble/sound strips Two-lane roundabouts Poor Rumble/sound strips One-lane roundabouts Unknown Pedestrian-actualized traffic signals at midblock One or two-lane roundabouts Good* Splitter island One or two-lane roundabouts Poor Yield signs One or two-lane roundabouts Poor Advanced vehicle detection technologies One or two-lane roundabouts Unknown *Simulation results only. This countermeasure has not yet been field tested. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data The figure below illustrates some of the roundabout elements that cause navigation difficulties for vision-impaired pedestrians. Vision-impaired pedestrians wait longer for crossable gaps because they cannot extend gaps that are initially too short with eye gazes and manual gestures in the same way that sighted pedestrians can. Traffic noise from inside the circle can mask sound cues from oncoming vehicles, especially quiet hybrid vehicles or vehicles coasting downhill HFG NON-SIGNALIZED INTERSECTIONS Version 2.0 10-10

Discussion Rumble/sound strips: One study (2) that looked at sound strips in two-lane roundabouts found that they increased the chance that a vision-impaired pedestrian would detect a stopping vehicle and also decreased the time needed to make the detection by more than 1 s. However, sound strips did not reduce false alarm rates, and the observed level of 13% false alarms makes this countermeasure unacceptable for deployment. One problem was that detecting a second stopping vehicle was particularly difficult if that vehicle was in the far lane because its sound was masked by the vehicle in the near lane. Participants in this study were not trained to use the sound cues provided by the pavement treatment, nor were they informed of the treatment before the debriefing. It is conceivable that with training, detection performance with the sound strips would have been better, and false alarms might have been reduced. However, the majority of vehicles stopped before reaching the sound strips and did not produce the intended sound, which may reduce the effectiveness of any training. This same study found that detection of stopping vehicles was relatively high when only one lane of traffic had to be monitored at a time (80% to 90% correct), which suggests that a sound-strip pavement treatment may be effective in single-lane roundabouts. However, any benefits of sound strips would likely be diminished if drivers consistently stop before reaching the sound strips, as mentioned previously. Pedestrian-actualized traffic signals at midblock: An analysis of pedestrian crossing treatments indicated that midblock signals appear to be a useful compromise between increasing safety/quality of service and reducing vehicle capacity (3). In particular, they appear to be effective in eliminating queues that back up into the intersection under most conditions. Midblock crosswalks also have the advantage of being farther away from the intersection traffic, which makes it easier for vision-impaired pedestrians to determine what is happening because there is less masking noise from traffic within the circle. Note, however, that these results are based on simulation research, and the results have not been studied empirically. Splitter islands: These features pose special challenges to vision-impaired pedestrians and are still associated with gap judgment difficulties. For example, one study found that vision-impaired participants (1) were nearly 2.5 times less likely to make correct judgments than sighted participants, (2) took longer to detect crossable gaps, and (3) were more likely to miss crossable gaps altogether (4). However, these differences were only significant at higher volume roundabouts. Overall, judging gaps in the exit lane is particularly difficult for vision-impaired pedestrians because they have to attend to vehicles in the exit lane and the circulatory roadway. This difficulty is compounded by drivers infrequently yielding in the exit lanes, typically because such yielding tends to back up traffic in the roundabout. Yield signs: Vision-impaired pedestrians typically failed to detect when drivers had yielded for them, which suggests that efforts to encourage driver yielding may be of limited use (5). Design Issues Because most vision-impaired and low-vision pedestrians rely primarily on sound cues to determine traffic conditions, quiet oncoming vehicles pose a potential hazard. In particular, hybrid vehicles operating in battery- powered mode and vehicles that coast downhill towards a roundabout can be difficult to hear above other traffic noise. In this case, providing sound strips or other measures that make these vehicles easier to hear may assist vision-impaired or low-vision pedestrians in detecting oncoming vehicles. Cross References Countermeasures for Improving Accessibility for Vision-Impaired Pedestrians at Signalized Intersections, 11-8 Key References 1. FHWA (2000). Roundabouts: An Informational Guide (Publication FHWA-RD-00-067). Washington, DC. 2. Inman, V.W., Davis, G.W., and Sauerburger, D. (2006). Pedestrian Access to Roundabouts: Assessment of Motorists Yielding to Visually Impaired Pedestrians and Potential Treatments to Improve Access. Washington DC: FHWA. 3. Rouphail, N., Hughes, R., and Chae, K. (2005). Exploratory simulation of pedestrian crossings at roundabouts. Journal of Transportation Engineering, 131(3), 211-218. 4. Guth, D., Ashmead, D., Long, R., Wall, R., and Ponchillia, P. (2005). Blind and sighted pedestrians’ judgments of gaps in traffic at roundabouts. Human Factors, 47(2), 314. 5. Ashmead, D.H., Guth, D., Wall, R.S, Long, R.G., and Ponchillia, P.E. (2005). Street crossing by sighted and blind pedestrians at a modern roundabout. Journal of Transportation Engineering, 131(11), 812-821. 10-11 HFG NON-SIGNALIZED INTERSECTIONS Version 2.0

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 600: Human Factors Guidelines for Road Systems: Second Edition provides data and insights of the extent to which road users’ needs, capabilities, and limitations are influenced by the effects of age, visual demands, cognition, and influence of expectancies.

NCHRP Report 600 provides guidance for roadway location elements and traffic engineering elements. The report also provides tutorials on special design topics, an index, and a glossary of technical terms.

The second edition of NCHRP 600 completes and updates the first edition, which was published previously in three collections.

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