Study of freeway traffic near an off-ramp

A bottleneck with a diminished capacity is shown to have arisen on a freeway segment whenever queues from the segment's off-ramp spilled-over and occupied its mandatory exit lane. Although the ramp's queues were confined to the right-most exit lane, non-exiting drivers reduced their speeds upon seeing these queues and this diminished flows in all lanes. It is also shown that the lengths of these exit queues were negatively correlated with the discharge flows in the freeway segment's adjacent lanes; i.e., longer exit queues from the over-saturated off-ramp were accompanied by lower discharge rates for the non-exiting vehicles. Whenever the off-ramp queues were prevented from spilling-over to the exit lane (by changing the logic of a nearby traffic signal), much higher flows were sustained on the freeway segment and a bottleneck did not arise there. These observations underscore the value of control strategies that enable diverging vehicles to exit a freeway unimpeded.