CE 3102
Fall 2008
Thought Problem 9
Light Traffic and the Poisson Process
A traffic engineer is designing a traffic signal for an intersection between a major street
and a minor street. One of the constraints that must be addressed is that on the minor
street there is a driveway 100 feet from the intersection, which should not be blocked by
vehicles queuing up during the minor street’s red phase. Traffic on the minor street is
light, with an average flow of 240 vehicles/hour, and a queued vehicle takes up 25 feet of
road. (a)
The engineer is considering using a 30-second red phase for minor street. What
is the probability the driveway is blocked? (b) A pedestrian is standing on the side of the
minor street, and needs at least a 20 second gap in the traffic to safely cross road. What is
the probability the next gap in the traffic stream permits a safe crossing?

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