Study of Freeway Traffic Near an Off-Ramp

Problem

An over-saturated freeway exit ramp that causes congestion in the exit lane also causes slow-down in the adjacent lanes, and can create a serious bottleneck on all lanes of the freeway.

Method

We observed the La Paz Road off ramp from Interstate 5 in Orange County using videotapes, a probe vehicle, and loop detectors (which worked intermittently)

Findings

In this particular case, the bottleneck did not seem to be caused by drivers squeezing into the exit lane at the last opportunity; this happened at a very low rate and rarely affected through-moving traffic. Non-exiting drivers simply seemed unwilling to drive fast while next to stop-and-go-traffic in the exit lane, which slowed all the lanes until they reached the off ramp.

When the traffic signal downstream of the off ramp provided right-of-way to exiting vehicles, no queue formed in the off ramp and no bottleneck arose; the traffic on the entire freeway had a very high flow. This particular traffic signal has a queue elimination strategy, triggered by a particular loop detector on the exit ramp. Such a strategy is very valuable, since it can prevent a bottleneck which might affect all lanes of the freeway. Where complete elimination of exit queues may not be feasible, these observations suggest that at least some improvement in freeway conditions can be realized by keeping these queues as small as possible.