Now consider the impact of double stoplights —
the dreaded ‘stoplight trap’
In this common situation, two stoplights are placed a few hundred feet apart, or at consecutive city blocks. When the first light turns green, the second light turns red shortly thereafter. When the first light is red the second light is green.

The result is traffic stopped on red at the first light is stopped again on red at the second light. During heavy traffic the second light receives twice as many vehicles as can get through the intersection. Traffic then backs up at the first light. Each time the lights cycle through green and red the traffic jam gets longer.

This stoplight configuration is COMMON and is a frequent cause of GRIDLOCK.

Rush hour expressway frustration
Now consider the impact of just one slow driver at rush hour on an expressway with two southbound lanes. Traffic behind the slow driver starts backing up, and as drivers react, each has less time to respond. Each subsequent vehicle must slow down more than the car it follows. Drivers in the backed-up lane start moving to the other lane to squeeze past the slow driver, causing a bottleneck that reduces the carrying capacity of both lanes. Eventually, traffic comes to a complete stop. This acts like a WAVE that moves further back in the line of traffic. Even after the slow vehicle exits the highway, this RIPPLE continues. After a few minutes the vehicles start moving again and eventually they are going full speed again. Drivers may have expected to see a wreck ahead, and are surprised to find no visible cause for this delay. Often the delay is caused by gawking — people slowing down to observe a driver pulled over on the side of the road by police.

This effect is seen repeatedly every day on I-4 traveling east from Orlando to Tampa and all drivers experience it in heavy traffic. It's why some cynical commuters have dubbed Interstate Highway 4 in Orlando the "I-4 parking lot."

Conclusion:
The busier a conventional expressway or road gets during rush hour, the fewer vehicles it is capable of carrying.

dreaded double stoplights
see animation

One slow car causes a traffic jam
enlarge image see animation

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