I’ve been driving for over 20 years, and I didn’t know this. I didn’t think any of them here in Springfield did. Now I’m wondering if they really are all timed, or if they really do have sensors and I just haven’t known it.
There was a popular video about “defensive driving” that recommended that you should stop at least 15 feet from the intersection or from the car in front of you, in case you are rear-ended. (In other words, don’t allow yourself to get pushed into the intersection or into the car in front of you.)
The video must have been in circulation for a long time, because I saw it in the mid-1970’s in high school and again in the mid-1990’s at a work-sponsored safety course. I thought it was bogus both times, but it’s possible that some people follow this logic.
Thats nice. Where is this?
In Atlanta, many side streets had them but as others have said major intersections were timed. However as a cyclist I would try to take low traffic routes which sent me over the sensors.
BTW I thought they used a magnet, not pressure.
I’m sure you’ve seen the induction actuated ones. They’re coil buried under the road and you can generally see the outlines. Here’s a bicycle oriented page with some pictures. I’m not actually sure what the pressure sensors looked like, they’re not used currently by anyone although some older ones may still be in use.
Bay Area. And yes, the ones here use magnetic fields.
I think some may be times, on bigger, busier streets, but even ones on small streets have sensors. Usually works well, except when the sensors go out.
Timed lights can be awesome - I used to be able to drive South on US1 from the NJ Turnpike exit in New Brunswick to Princeton without hitting a single red light (18 miles) if the traffic was okay and I was in the groove. There were probably 50 lights in that stretch.
Maybe I’ll have time tonight to compute how fast a car would be going to push another car, with brakes on, 15 feet. You usually have a couple of feet for the crosswalk, so 15 feet beyond that would be about 20. That seems way too far.
I think most of the lights in my town are induction actuated - you can see the slots cut into the roadway for the sensors before you even get up to the intersection.
I often see people sitting at a light that should have changed for them but hasn’t bercause they have stopped completely past the stop line and are sitting completely in the pedestrian crosswalk. Since they went past the sensor, the light thinks they have left the intersection and doesn’t know anyone it sitting there. I roll up, stop behind the big, wide obvious stop line and suddenly our section of the intersection doesn’t get passed over in the cycle.
We have one area traffic light that’s sensor-activated and also has a big sign telling people to pull up to the white line to activate the light.
There’s only one other such traffic signal I know of, and there’s no sign or other indication that it’s sensored (the tipoff is that you can be approaching it on red and if you don’t get there in time the green light cycle doesn’t activate in the expected sequence).
I don’t remember this in driver ed, but they did warn us when advancing into an intersection on green prior to making a left turn, to keep the car’s wheels straight - because if you start rotating the wheel left, a car rear-ending you will push you into oncoming traffic.
They don’t look familiar, but I’ll keep an eye out for them when I go driving this week.
Sensors? I encounter people every day who’ve lived here their whole lives, decades and decades, people who were born here and are now middle-aged, who don’t know to keep to the right when walking down a hallway or a sidewalk, or that you wait for people to get off an elevator before you get on it, or that that rubber thing is to separate your groceries from the guy behind you’s groceries.
Or that you aren’t supposed to block an intersection with your car or that there’s no good reason to pull up as far as you can in the left-turn lane when the light’s red, unless you’re just that big an asshole that you won’t let the guy in the right lane, who can legally turn, see around you.
Get those people straightened out, then teach them about sensors.
I was rear-ended at an intersection a couple years back; the impact both pushed me across the crosswalk and the bump jostled my foot off the brake pedal; With the momentum imparted by the impact, by the time I’d realized what was going on I was halfway through the intersection. (Fortunately the light had only just turned at the time of impact, and the intersection was clear.) I was at a dead stop with my foot resting on the brake prior to the impact.
Of course, I was also hit by a goddamn garbage truck, a huge mother of a thing. Three times longer than my car if it was an inch, and two and a half times taller. I really don’t think it was going all that fast on impact (it left rubber for fifty feet behind it from trying to stop) but suffice it to say that you must remember to account for mass of the impacting vehicle into that computation of yours, (and a possible brief cessasion of brakes on impact).
'Round here, the sensors are easy to spot. Look for the large rectangular shape carved in the pavement. Often looks like someone took a concrete saw and tried to cut out a block (overrunning the corners and all). Actually, it’s just to lay down the sensor line.
These ‘sensors’ are often just big, crude metal detectors. All it takes is a large loop of wire, with a bit of current, and anything metal will disrupt the flow…in this case, cars passing over it. Much easier to install than pressure sensors, especially after the fact (that’s just judging by my own area).
This said, many lights have timers, too, especially busy high-flow intersections during rush hours.
As mentioned upthread, not “anything metal” will do it. It has to be a ferrous metal (steel), and it has to be massive enough to trip the sensor. Some motorcycles don’t have enough steel to trip those types of sensors.
That’s wrong, it just has to be something that conducts electricity.
No sensor-activated traffic lights in my town - everyone is timed, though from late at night to early morning the lights at some intersections blink yellow.
One thing that may cause people to stop too far back is improperly placed traffic lights. If a tall driver is driving a low to the ground car stops at the stop line she or he ends up leaning forward, crouching over the steering wheel and peering up at the light. If you stop further back this doesn’t happen.
I’ve noticed in France, land of really small cars, small auxiliary traffic lights mounted on the streed corners just about eye level. It seemed like a good idea to me.
‘With brakes on’? With your foot? If somebody crashes into you, your first instinct isn’t going to be to keep your foot down. It’s going to be to bring your feet upwards. The same reasoning was given to me when learning in a manual (i.e. the British norm), that when at a red light you should always always engage the parking/hand/e-brake (let’s not get into that one), specifically because it’s the best way of keeping the car still if you’re hit. Whether you’re holding first gear with the clutch, or in neutral, or whatever.
There was a particularly bad intersection near where I used to live in the Twin Cities. As you approach the lights from the east, there was a set of train tracks about two car lengths from the intersection. The stop line was on the opposite side of the tracks from the intersection (makes sense, since you don’t want cars stopped on the tracks).
The only problem was that the sensors for the light were between the tracks and intersection, so you had to pull across the stop line to trigger them.
My dad does that all the time, and it drives me nuts. When I asked him about it, the response is either “I"m in no hurry” or “This is where I stopped.” :rolleyes:
Depending on the design of the sensor, non-ferrous metal can be detected, as it lowers the inductance of the sensing coils (shorted turn effect) BTW, attaching a magnet to the bottom of a motorcycle has exactly the same effect on the sensors that attaching said magnet to your fuel line will have on your gas milage, which is to say zero…doesn’t seem to stop people selling them for either purpose though.