Human Factors Guidelines for Road Systems 2021 Update, Volume 1: Updated and New Chapters (2022) / Chapter Skim
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Pages 27-37
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From page 27... ...
HFG NON-SIGNALIZED INTERSECTIONS VERSION 2.1 10-1 CHAPTER 10 NON-SIGNALIZED INTERSECTIONS Acceptable Gap Distance ........................................................................................................... 10-2 Factors Affecting Acceptable Gap .............................................................................................
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From page 28... ...
HFG NON-SIGNALIZED INTERSECTIONS VERSION 2.1 10-2 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, or crossing a major road, that provide sufficient time for the minor-road vehicle to accelerate from stop and complete a turn or crossing 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)
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From page 29... ...
HFG NON-SIGNALIZED INTERSECTIONS VERSION 2.1 10-3 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)
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From page 30... ...
HFG NON-SIGNALIZED INTERSECTIONS VERSION 2.1 10-4 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., inexperienced drivers, older drivers) to accept smaller or larger gaps than they would otherwise accept under normal conditions.
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From page 31... ...
HFG NON-SIGNALIZED INTERSECTIONS VERSION 2.1 10-5 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)
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From page 32... ...
HFG NON-SIGNALIZED INTERSECTIONS VERSION 2.1 10-6 LEFT-TURN LANES AT NON-SIGNALIZED INTERSECTIONS Introduction Left-turn lanes at non-signalized intersections refers to auxiliary lanes that minimize the impact of left-turning traffic on through traffic while reducing crash risk. Left-turn lanes are effective at reducing crashes and improving traffic flow at intersections because they route left-turning vehicles out of the through lane and provide space for them to decelerate and wait for a gap in traffic before turning.
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From page 33... ...
HFG NON-SIGNALIZED INTERSECTIONS VERSION 2.1 10-7 Discussion Left-turn lanes provide both safety and performance benefits for left turns at intersections. Common reasons for considering installation of a left-turn lane include: speeds that are too high to make left turns safely, intersections with high crash occurrence involving left-turning vehicles (e.g., rear-end or sideswipe crashes as turning vehicles interact with through vehicles)
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From page 34... ...
HFG NON-SIGNALIZED INTERSECTIONS VERSION 2.1 10-8 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)
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From page 35... ...
HFG NON-SIGNALIZED INTERSECTIONS VERSION 2.1 10-9 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.
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From page 36... ...
HFG NON-SIGNALIZED INTERSECTIONS VERSION 2.1 10-10 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)
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From page 37... ...
HFG NON-SIGNALIZED INTERSECTIONS VERSION 2.1 10-11 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.
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