Push Button, Cars Stop, Emergency Vehicles Go

Yesterday we discussed the button on a pole that pedestrians use at the crosswalk (zebra crossing). It makes the lights change sooner, and stops the cars a bit longer so pedestrians can walk across.

Today we'll talk about a different button. A button in a bus, fire truck, ambulance or police car. It's called the traffic signal preemption button.

For a bus, this magic device can HOLD a signal GREEN a bit longer, so buses move more quickly through heavy traffic. The bus system delivers a message to a sensor at the traffic signal. It's like shouting Hold that door! as you run towards an elevator.

A slightly-different device is in emergency vehicles to speed their response to an accident or fire. If the signals in front of the vehicle are GREEN, the emergency vehicle can go more quickly and safely. This type of device CHANGES a signal from RED to GREEN. 

Once the traffic signal has changed all other directions to RED, it gives the emergency vehicle a bright white light to show that all is clear. As soon as the traffic signal it loses its traffic preemption notice, the lights return to their normal pattern.

How does it work?

There are many systems. One uses bright strobe lights flashing at 10-14 cycles per second. Another depends upon an infra-red signal, like your TV remote. It's directed up and forwards towards the traffic signal. Another system uses a limited-range radio transmitter which isn't as susceptible to fog, rain or other atmospheric conditions that could block the visible or infra-red lights.

A more complex model has the vehicle communicating with the lights, and the lights warning those along the route that a vehicle is coming. That takes real intelligence! It's covered in patent 7116245, in case you care to read up on it ... here's one of the diagrams.

Yet another system relies on a GPS location signal emanating from the vehicle. It's covered under patent 7327280. With this approach, a central depot can see the vehicle moving along a screen and change the signals for the emergency vehicle. This system is also at the mercy of signals bouncing around through tall buildings, trees, etc.

Note - Both these systems were invented by Aaron Batchelder, of CalTech in Pasadena, California.

Traffic signal preemption is used at railway crossings too, to make sure all vehicles have cleared a set of railroad tracks and that no new vehicles are allowed to approach too closely. If possible, the signals allow nearby non-crossing traffic to keep moving, even "out of cycle" from the normal pattern.

I know pedestrians are wondering - if you are anxiously pushing a button at a crosswalk, do you have to wait for the emergency vehicle?  YES. It takes priority over your request, as well as priority over the cars.

You might be the impatient type who always wants a GREEN light, but I would advise you NOT to try to buy or build your own light-changing device. It is illegal for drivers or pedestrians to use them.