When placing traffic lights at an intersection, traffic planners have to determine how long the yellow light should stay on. This
Time will depend on the speed limit on the road and on the length of the intersection. Outside our school, the intersection at
Bob st. and Wall st. has a traffic light. For cars travelling along Wall st. the speed limit is 50 km/h. The distance to
Cross the intersection is 34 m. Assume the length of a typical car is 4.5 m. For safety reasons, the Ministry of Transportation
Recommends vehicles stop with an acceleration no higher than 3.0 m/s2 [backwards]. Your job is to determine how long the
Yellow light should be at the intersection. Your calculations should contain the following:
1. How much distance (in m) will it take a car to stop when a light turns yellow if it is travelling at the speed limit of 50
Km/h? Drivers do not react right away when they see the light change. Human reaction times vary from 0.50 s to 1.0
s. In your calculation, assume the driver does not react for 1.0 s. In other words, the car continues to travel at the
Speed limit for 1.0 s after the light has turned. Then the driver brakes to a stop at an acceleration of 3.0 m/s2
[backward]. What total distance will the car need to stop including reaction time?
2. Using your result from above determine how long the yellow light needs to be to allow vehicles that are too close to
Stop to continue at the speed limit and clear the intersection before the light turns red.
3. Discuss your results. Modern car tires allow cars to stop at accelerations of 10.0 m/s2 [backwards] without losing
Control. This would allow you to shorten the yellow light time. Is this a good idea? What other factors will determine
The stopping distance of a car?

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