10.2 Acceptable Gap Distance

Motorists turning from a minor road onto a major road will not make a turn until they observe an adequate gap in traffic on the major road. Acceptable gap distance is the size of gap that provides motorists enough time to stop and complete a turn without interfering with traffic. Intersection sight distance in both directions must equal the distance traveled at the major road’s design speed during period equal to the gap.
- A gap is the time interval between successive vehicles. It is measured from the rear of the lead vehicle to the front of the trailing vehicle
Table 10.1 provides baseline guidance for acceptable time gaps. Values assume the stopped vehicle is turning onto a two-lane highway with no median and a grade ≤ 3%.

Table 10.1 Acceptable Time Gaps
Design Vehicle	Time Gap (tg) at Design Speed of Major Road
Design Vehicle	Left Turn	Right Turn
Passenger Car	7.5 seconds	6.5 seconds
Single-Unit Truck	9.5 seconds	8.5 seconds
Combination Truck	11.5 seconds	10.5 seconds
For turns onto multilane roads, add 0.5 seconds to the time gap for passenger cars and 0.7 seconds for tucks for each additional lane For left turns onto minor roads where the approach grade is an upgrade > 3%, add 0.2 seconds for each 1% increase in grade. For right turns onto minor roads where the approach grade is an upgrade > 3%, add 0.1 seconds for each 1% increase in grade.

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Chris VanDyke

Research Scientist | Program Manager

chrisvandyke@uky.edu