One of the biggest annoyances with driving a car is, when approaching an intersection, and the light changes from green to yellow, is deciding whether to stop or to speed through. Often the choice is obvious, but all-too-often it isn't, and accidents sometimes result. --Admittedly, accidents from this cause are somewhat rarer than they used to be, because most intersections nowadays have the yellow turning to red, and both directions seeing red, before one switches to green. But the uncertainty and annoyance remains!
Let the Traffic Engineer assume a car is travelling at (or even a little more than) the speed limit. He knows the average distance that it takes for an average vehicle to stop -- NOT a panic stop! -- from that speed. So suppose this distance is measured away from every intersection, and a yellow line painted across the appropriate lanes.
Drivers encountering this line can know that if they haven't reached it when the light turns yellow, then they have time -- and room! -- to stop. If they have reached the line (can't see it through windshield in front of car), then the driver can be confident that the light will stay yellow long enough to get through the intersection, before it turns red.
Towns that try to trap passers-through (by cycling light from green to red too fast for anyone to stop) will be out of luck. Tough!-- Vernon, Jun 08 2004 I think most lights are set up so that the duration of the yellow light corresponds to the posted speed limit (if you are going fast you need more warning time to stop, hence the yellow lasts longer). And although I'm sure many small towns are more than happy to take your money as you pass through, I doubt they would intentionally try to get people to run reds. That's just dangerous. It's more likely that you have failed to slow down to their posted speed limit and as such don't have enough time to react to their slow-speed (quick duration) yellow lights... Terrific idea by the way. (+)-- luecke, Jun 08 2004 First, [+] for intent and thinking outside the oven. Second, do you think that this would result in people accelerating to the yellow line in an attempt to cross it soon enough to get through the yellow light? That would seem to compound the problem you're trying to solve. (?)-- contracts, Jun 08 2004 Exactly which average vehicle are we talking about? I foresee all sorts of rangles (legal or otherwise) ensuing about the exact positioning of the line and whether or not a car had crossed it before they crashed the lights. If a car driver is unable to judge his stopping distances correctly then he/she shouldn't be driving.-- DrBob, Jun 08 2004 [luecke], I did specify that the speed limit be used when deciding where to place the line. A speed-trap town (where highway speed limit suddenly reduces to residence-area speed) isn't quite the same as a run-the-red-light-trap town.
[contracts] and [DrBob], both of these issues are partly handled by my specifying "NOT a panic stop!". A casual stop takes a lot more space than a panic stop -- and auto makers know just how quickly their vehicles are designed to slow down, without excess wear-and-tear on the brakes. So the yellow lines are very likely to be far enough from intersections that persons who speed up enough to ensure they get through a changing light can probably be charged with SIGNIFICANTLY speeding over the limit. And if the "average" was computed by including some heavyweights like fully-loaded 18-wheelers, then much legal loopholing goes away, because that skews the average in favor of safety.-- Vernon, Jun 08 2004 Fix a nagging societal ill with some computing power and paint? +-- dpsyplc, Jun 08 2004 I read, not too long ago, that most yellow lights are about 4 seconds in duration (around here anyway). As Vernon mentions, some municipalities will alter certain intersections to have shorter yellow lights at certain times of the day to collect citation revenue. The lights in most places are computerized and can vary their behavior on a schedule or be remotely altered to respond to a cirumstance of traffic.
The point is that painted yellow lines would effectively prevent the light dwell times from being changed and, so, I think many cities would opt not to paint them.
Is there a way that these could be dynamic? Projection? Or akin to the runway approach VASI lights that change color (white [or amber]=too high, red=too low, green=on glide slope)?-- bristolz, Jun 08 2004 [bristolz], I think that most automated changes to intersection lights involve the time they spend as red or green, and not yellow. The speed limit doesn't change, see, so autos still need the same distance to stop.-- Vernon, Jun 08 2004 I know the City of Redmond engineer in charge of, amongst other things, traffic lights. I'll ask him. I do think, though, that the yellow dwell is altered.-- bristolz, Jun 08 2004 //If they have reached the line (can't see it through windshield in front of car), //
depends on what kind of a car you're driving. if you're driving something like a bus, or a pickup drive, you can see it when you're closer to the stop-line than other drivers in smaller cars; so you take for granted the fact that you can safely stop, although your stop is more hazardous than planned. the stop-distance depends a lot on the initial speed. if that's much more than the speed limit (I usually go almost twice as that in some pedestrian-free areas) the safety-stop-line would be rather miscelaneous..
experience makes you realise if you have or don't have time to stop safely before the stop-line anyway. if you can't, you accelerate, if you can stop, you g r a d u a l l y slow down-- sweet, Jun 08 2004 [bristolz], it is possible that during rush-hour, traffic flow normally slows so much below the speed limit that a reduced yellow interval makes sense. Perhaps more simple than an adjustable line, is to proclaim that the line should not apply during rush hour. Because if the traffic is going slower and the light changes to yellow, those slowed drivers in front of the line will mostly KNOW they still have plenty of room to stop. A second yellow line, for rush hour only, is also a possibility.
[sweet], the distance in front of your vehicle, where you can no longer see the yellow line, is a trivial fraction of the distance of that line from the intersection. It will remain true that if you are not going much over the speed limit, you will have time and room to stop. If you normally speed excessively, then, sorry, but you are risking more lives than your own, and this is why the cops will be after you.-- Vernon, Jun 08 2004 Add a red line farther back, so that if one hasn't crossed it when the light turns to red, driving at the speed limit means arriving at the intersection after it turns to green and no need to slow down.-- FarmerJohn, Jun 08 2004 [FarmerJohn], that's not so practical, since often there will be cars stopped because of the red light. They will require time to accelerate after light changes to green, and they will be in the way of any late arrival that has not slowed down.-- Vernon, Jun 09 2004 Yellow light times are generally computed using the ITE Kell & Fullerton formula which takes into account speed, grade and perception time.
And FamerJohn that is very brilliant, but it is rare when you can see the light from the distance where the red (green) light would be placed.-- haywardt, Feb 17 2012 random, halfbakery