Impact of single traffic light at rush hour
Traffic signals seem like a reasonable approach. But the reality is that traffic is at a standstill in all directions most of the time. The IMPACT of a single traffic signal on a highway or street is to drastically reduce the maximum traffic carrying capacity of that road. During rush hour traffic is always backed up when a light turns green, because more vehicles are arriving at the stoplight than can get through it. In a case study, on a road designed to carry 12,000 vehicles per hour at 45 mph, a single traffic light reduced roadway capacity by 70 percent to 2,520 vehicles per hour for westbound traffic.

Here is why.

The right-of-way time (green light) must be "shared" by traffic in all directions, and provision made for amber signals to warn drivers the light is about to change to red. In the example used above, recorded during rush hour at a 4-way intersection with two lanes in each direction for through traffic, and four left-turn lanes, the entire cycle of the traffic light took 182 seconds (see illustration 5). During this time the light was green for through westbound traffic for only 39 seconds (12.6 seconds out of every minute).

But that’s only part of the problem.

What happens when the light finally turns green?
Seconds pass as the first vehicles slowly crawl into the interchange and accelerate. Some current vehicles have gauges which show how many miles per gallon (MPG) the vehicle is achieving at any given time. During initial acceleration, the engine must propel the dead weight of the 2,500 to 5,000 pound car from a complete standstill. Engine speed (RPM) rises dramatically as gasoline pours into the engine. The result: a fuel efficiency of 5 to 10 MPG! This highly inefficient use of fuel occurs at every traffic light in the country, every day of the year.

In addition, consider the billions of barrels of gasoline burned sitting at traffic signals; all the extra pollution; wear on the vehicle’s brakes at each stop; and wear on transmissions with each acceleration. Finally, have you considered whether traffic signals are SAFE?

Practical experience shows us that frustrated or impatient drivers, instead of slowing and stopping when a light turns amber, actually ACCELERATE in order to get through the light, and sooner or later, cause accidents and carnage

Finally, consider the danger to pedestrians using "walk" "don’t walk" signals at traffic-light-controlled intersections. Typically, the "walk" signal is very brief, and starts to go amber while pedestrians are half way across the intersection. The pedestrian must exercise blind faith in motorists who may run the red light, dash across the amber light, or turn right on red while paying scant attention. For a young pedestrian with 20:20 vision, strong legs and great reflexes, crossing the road may seem an exciting sport. But for anyone who is elderly, infirm, or has poor vision, or is simply not fully alert, "walk" "don’t walk" can be a frightening, nerve-wracking and perhaps deadly experience.

To return to our traffic light example….
Gradually, after the first vehicles have accelerated through the traffic-signal-controlled interchange and accelerate, following vehicles start going faster, and get through the intersection at increasing speeds. But no vehicle goes through without stopping for one or more cycles of this stoplight. The capacity of the roadway is limited by how many vehicles can get through the light while it is green, and these vehicles are all traveling at much less than the design speed of the roadway. Each succeeding light will similarly cause delay because traffic will arrive at near design speed, then slow down to the rate it can get through the intersection. The capacity of the highway is much less affected by the addition of subsequent stoplights. That’s because the first light causes a lesser traffic flow for subsequent lights (but each stoplight causes cumulative delays for the individual driver). This is a simplified analysis of a very complex equation.

It is also why a 5-mile commute on a major roadway with several traffic lights can take 30 minutes or more to travel during rush hour. In the ATS system, where there are no traffic lights and through traffic never has to stop while vehicles enter or exit the roadway, a single lane can carry 18,000 vehicles an hour.

the waiting game

Illustration 5
The Trouble with Traffic Lights
see animation

In the example above, through traffic
only had a green light 12.6 seconds
out of every minute. This resulted in
a flow rate of only 2,520 cars per hour.

Traffic lights: A BOTTLENECK that cuts traffic flow

"No advanced transportation system should be considered viable if it requires vehicles to stop at any time other than for departures, arrivals or stops for the travelers’ convenience"

—KATS Inventor Waldemar Kissel

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