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Gap Acceptance

Gap acceptance is defined as the probability of pulling out from a side road or turning left in front of approaching traffic based upon the distance and speed of the approaching vehicle (potential hazard).

The gap itself is a time measurement.  Distance alone doesn’t provide sufficient information for a driver to make a decision on whether or not it’s safe to proceed.  A driver needs to know how much time is available to clear the path of the approaching vehicle before it arrives.  Research has demonstrated that approximately 50% of drivers will attempt a maneuver if the gap is at least 5.5 seconds.

Since velocity is nothing more than a change in distance per unit time, we can calculate the distance required for this gap on a 35 mph roadway.  At 35 mph, a vehicle travels 51 ft every second.  At 5.5 seconds, a vehicle on a road with traffic flowing at 35 mph would need to be 280.5 ft away for half of the drivers to attempt their maneuver.

But what happens when that vehicle is traveling faster than the flow of traffic?

If the approaching vehicle is actually at 45 mph or 66 feet every second, it only takes 4.25 seconds to reach the collision point.  With a gap of 4.25 seconds, less that half of the drivers would have pulled into the path of the approaching vehicle.

What does this mean? Because the vehicle was speeding, it convinced the driver that there was enough time to proceed safely.  When you get to the 50% threshold, the additional 10 mph made it more likely than not that a driver would have attempted their maneuver or “accepted the gap.”

When you have a “failure to yield” case, you should consider gap acceptance when conducting your analysis.  The Monte Carlo analysis products we offer include this analysis on all of these types of collisions.


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