Anther reason t oflash the light? Change the stop light from red to green.

[pbs911]

I've noticed that police sometimes flash their high beams when comming to a signaled intersection when they are approaching a red light. It seems the light goes green for them. I always thought that signals were controlled by putting weight on panels bwlow the asphalt, and as for emergency vehicles they had some sort of "automatice signal changer." Anyone know if there is any truth to any of this.

As a Porsche driver (on topic) I believe we should have priority at all intesections and if there is a way to change a red light to green, then all Pcars should be so equipted.

[widebody911]

Yes. On some signals, there is a special 'receiver' which can be triggered by a special light on the vehicle. I don't think standard headlights will trip it, otherwise it would be going back and forth incessantly!

From http://www.roadtripamerica.com/rw/rw40.htm

There are a couple of systems in existence that, if they were fully developed and
implemented, might one day provide the kind of information that we so desperately
sought that afternoon in July. The programs are known as Safety Warning System
(SWS) and Safety-Alert System (SAS). SAS is a proprietary program of Cobra
Electronics Corporation, the makers of the #1 CB radio for truckers and a very
successful line of radar detectors. The SAS is currently operating in its most basic
mode for most of the country. The SAS targets the strobe light system that is
installed on the light bars of hundreds of emergency vehicles and traffic control
signals throughout the United States. When an emergency vehicle uses the strobe
transmitter to change the traffic light to green, an alert flashes on the radar detector
indicating that an emergency vehicle is nearby.

[Superman]

This is a pretty minor point, but the signal system does not sense a vehicle's weight. The thing that's under the asphalt is a "wire loop" and it senses the vehicle's electromagnetic field. True story.

[widebody911]

Yup, and it's a problem with motorcycles and some smaller cars, which don't have big 'field presence'

[island911]

And for a pretty minor² point, if you notice these "under the asphalt is a wire loops" on the freeway, in tandem pairs, they are for checking speed. . .aka "Traffic Flow."
The fact that they happen to be coincident with traffic cameras, well that I'm sure is *just* a coincident.

[BlueSkyJaunte]

They're so common here in the Phoenix area that some joker started selling "black boxes" guaranteed to change the light for you....complete hoax, of course....

One day I sat at a "wire loop" on my motorcycle waiting to make a left turn through 4 light changes. Eventually just went straight (had I made the left through the red a cop would have been sure to drive up just then). Yes, there are jokers also selling devices guaranteeing to make your bike "visible" to the loops....

[stormcrow]

You are correct about the flashing light to change from red to green. Some traffic light signals are set up this way. Emergency vehicles in some parts of the country have strobes on their vehicles. As they approach the intersection with their strob flashing, it sends a signal to change the light from red to green

I have also read that some public trans busses will have this capability to speed up their routes.

Steve

[juanbenae]

i work in the public works field and if i may for once chime in with some real info, as opposed to my stadard smart ace remark.. the loop bit by superman was in fact correct. some agencies are concertrating wires at the 1st spot at an intersection to accomidate motorcycles and bicycles.

the flashing of the head lights is an automatic portion of the siren/lightbar deal on cop cars, (i drive a retired ford crown vicky cop car all day at work) it is all part of the xmas tree look they aspire for. the sensor mentioned is correct on the signal pole and if you look close on emergency vehicles you will see a much brighter yet smaller strobe light affair up front flashing when approaching a signal. the city i work for requires them to be supplied by signal installation contractors when installing or retrofitting signals. i have to this point not been able to get my hands on one,, or find the unit on one of my departments retired cop cars. sucks for me.

[Dantilla]

Around here, I believe that the strobe turns all the lights red so that everyone at an intersection is at a standstill for the emergency vehicles.

I don't want that on MY car!