When Waiting Turns Into Risk
In the last issue, we looked at pedestrian crossings as more than simple counts—focusing on how long people wait, how they enter the roadway, and how gap availability shapes what they can do.
The same situation shows up in another place on the network: drivers waiting on a minor road to enter a busy arterial.
At stop-controlled intersections, drivers wait for a gap in traffic before they can turn or cross. Some gaps are easy to use. Others are not. And those conditions change depending on how traffic is moving on the arterial.
When traffic is lighter, larger gaps are more likely to appear. As traffic builds and vehicles move closer together, those larger gaps become less common.
At one location we analyzed, average gaps fall below what’s typically sufficient for a comfortable, safe entry for much of the day.
When fewer gaps are available, drivers wait longer to find one they can use. As those waits increase, vehicles begin to queue on the minor approach.
Waiting drivers begin edging forward into the intersection.
As the queue builds, the pressure felt by the driver to go increases, and the likelihood of accepting smaller gaps increases with it.
It is a condition where the available opportunities and the demand to use them begin to work against each other and this condition can be identified and understood before a crash ever occurs.
Just as with pedestrian crossings, the most important insight is not simply that a movement occurred, but what had to happen for it to occur.
In the next issue, we’ll look at another kind of gap: what crowdsourced data misses about how streets are actually used and how combining it with observed behavior helps tell a clearer story.