Half a croissant, on a plate, with a sign in front of it saying '50c'
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Dynamic Speed Limits

Change the speed limit based on how fast traffic is moving
  (+14, -7)
(+14, -7)
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The safest speed to drive is the same one that most of the traffic around you is doing. Yet today, that's often illegal because speed limits are set once and don't change.

It would be possible to create a dynamic speed limit by adding only a couple of components to each speed limit sign. You'd need a changeable display, a radar/laser/whatever speed sensor, a modest processor, and a way to communicate with the speed limit signs up and down the road (small laser or other wireless connection).

If each sign uses the average speed it measures (or a default) and the speeds reported by the two signs closest to it, it could display a speed limit which changes dynamically based on actual safe speeds to drive in traffic.

Tickets could still be issued for unsafe driving, or for exceeding the speed limit by x%.

5tonsofflax, Feb 22 2002

Relative Velocity Speed Cameras Relative_20Velocity_20Speed_20Cameras
[quantum_flux, Mar 14 2008]


       Huh? So if everyone all drives faster, they know the speed limit will go up? How is that a good thing?
waugsqueke, Feb 22 2002

       The ramjet theory: the faster it goes, the faster it goes. Not very practical for safe travel but, hey, it's an idea.   

       The citations for speeding would be interesting as they might read something like, "77 in a prevailing 65."
bristolz, Feb 22 2002

       Since people usually tend to ignore more and more signs as they learn their daily commute, I think this would backfire into another cause for driver-frustration.
waugs, that's effectively the way it works in Michigan anyway, were we all speed in the right places, and completely ignore whatever the posted speed is. Rule by majority.
RayfordSteele, Feb 22 2002

       An absolutely excellent idea, 5tf. In peak hour or in adverse conditions, the speed goes down, and goes right up when the road is clear on a sunny weekend afternoon. When can we start?
jetckalz, Feb 22 2002

       I'm with RS on this one. This all assumes you actually pay any attention to the posted limit. In a perfect world we'd all travel at the appropriate velocity for the conditions at hand. They can't ticket ALL of us.
rbl, Feb 22 2002

       Rule by majority is exactly what I want here. This is just a way to formalize it and make it legal.   

       Jetckalz seems to have picked up on the advantages of having a changeable speed limit.
5tonsofflax, Feb 22 2002

       Wouldn't this *compound* the problem of traffic jams? The reason traffic backs up is that the road doesn't have the capacity to handle the cars at the prevailing speed. The two solutions are less cars or higher speeds. Given a fixed number of cars, the *only* solution is higher speeds (safety is a different matter). So once prevailing speeds fall below the throughput capacity of the road, the traffic jam would be made progressively worse. So the converse of the ramjet principle is true, too: the slower it goes, the slower it goes.
beauxeault, Feb 22 2002

       If speed limits weren't intended as revenue generators, it wouldn't be necessary to enforce the posted limits per se; the government could instead issue citations for those who drove unsafely fast. Being at the posted limit during reasonable conditions, moving with traffic, or being reasonably close to the limit during perfect conditions would all constitute prima facie evidence that the driver's speed was reasonable; losing control due to excess speed would constitute prima facie evidence that the driver's speed was not reasonable.   

       Of course, it's much more profitable for cities to set speed limits far below what the roads could safely handle and then ticket people who deign to drive reasonably.
supercat, Feb 23 2002

       I don't know about other places, but in California the speed law is to go is fast as it is safe to go. This means that you can get a ticket for going 45 in a 45 zone if the cop thinks it isn't safe. I think this is a good idea.
timbong, Feb 23 2002

       Think your assertion is wrong, supercat...Speed limits in most places (especially highly urbanized areas) are not intended as revenue generators. Usually they are established to protect the greatest number of people under expected conditions the greatest part of the time...Which continues to seem like a sensible approach to me. In the meantime, get off my rear bumper; I'm already going as fast as I think traffic and conditions allow.
jurist, Feb 23 2002

       I have been told and observed that in Austin when they build a new road they leave it unmarked for a while, measure traffic, and set the speed limit to the 80th percentile.
tolly3, Feb 23 2002

       Timbong, you can still get a ticket for exceeding the posted limit, whether or not everyone else is too. If you're the one they catch, you get a ticket.   

       Jurist: Then get out of the left lane, you're blocking me. <grin>   

       Tolly3, that would be the SENSIBLE way of doing it, so it's probably wrong...
StarChaser, Feb 23 2002

       I don't know about America, but here in England variable speed limit signs have been in use for some time in order to combat congestion. If people had actually obeyed the signs, it might have worked, but noone seems to care.
joarvat, Jun 18 2002

       //and set the speed limit to the 80th percentile//
//would be the SENSIBLE way of doing it, so it's probably wrong//

       In Australia, the Australian Standards Association (the manuals of which are used in Engineering and for related legal battles in Australia) advocate using the "85th percentile" for speed limits. That is the speed at which 85% or less of the traffic travels. Further it is thought that a speed deviation of 20km/h(12.4mph) is six times more likely to cause a traffic accident. In West Australia, new speed limits are supposed to be determined by the 85th percentile. Sometimes, however, politics gets in the way.   

       In my opinion, the speed at which a person drives is, partly an environmental effect (the environment being the road and verges), partly confidence (low- and over- included), partly training, partly legal and partly habit. The environmental factor is caused by the amount of clutter and possible danger that is perceived consciously and unconsciously. Note I say perceived, not actual. So if the road is slippery, narrow, with a lot of driveways and trees it is likely that a person will drive more slowly than if they are on a wide, multi-laned, low trafficed Highway with no driveways and a wide clear verge. I think in truth, if they wanted to achieve lower speeds on the roads what they really need to do is narrow the effective width of the road and introduce more "clutter" on the verges. This actually works, by the way, you just put up more trees and narrow the effective road width with longitudinal parking or edge paint. The problem is that people then complain because it is "unsafe". They tend to forget this when you widen the roads, complaining about the people "speeding" down the road. This is usually when they demand Traffic Control Devices such as humps and mini-roundabouts, which I think are more unsafe than the narrow roads.   

       Damn, I'm lecturing (again). It is likely that this would work to a certain degree. I don't think that you would get much of a change in the 85th percentile, because I think much of that is a psychological thing, so the speed might only alter according to conditions. Whether a person is speeding would become a personal judgement thing however and I can't see the police/"legal system" being happy with that. The speeds would probably not increase at peak hour, that has more to do with road capacity and traffic volume, ie when everybody wants to go home there's too many cars and not enough road capacity. The best way to alleviate peak hour traffic loads would be to change business hours so that everybody was working more flexible hours (Say 3 eight hour shifts where the middle shift worked overlapping both the early and late shifts).
CrumbsDM, Oct 07 2002

       I drove in Australia, (there too left is the right side) and was surprised to see speed signs every 100 meters. Before a bend they tell you 40 (Km/H) and then back to 100 and even 110. In Israel and in the US I never encountered so much bother about the speed signs since everybody simply tend to ignore them.   

       (An old book by Petzold taught how to right a graphic user interface for the Dos' old Abort/Retry/Ignore: The ignore was a speed sign icon, the Retry was a gambling machine and the Abort was obscured by some dialog box.
pashute, Nov 04 2002

       Changing speed limits are thoroughly baked (for over 30 years to my knowledge) on the New Jersey Turnpike. When weather conditions demand extra caution, lighted speed limit signs come on that set the limit lower for safety. Of course that's not what you're after here, but it is related. Tried to find a picture of one on the web to link but was unsuccessful. (WTAGIPBAN)
krelnik, Nov 04 2002

       All of the electronics are out there. LED traffic lights tell you how fast to GO. Start - stop traffic makes me crazy, turns winter roads to ice, wastes energy and creates opportunity for colissions. In july, fine weather, travell Crowchild at 88km.Speed posted on Green light.( Don't need rows of sign posts ) In October while everyone is learning to drive again, perhaps travell at 73km. Construction sites 50km. Whatever the speed or time of year, the next set of lights knows when you will arive and posts the appropriate speed for that section of road. The prize for being responsible, a safe, flowing relaxed journey. Drive like an idiot, pay huge fines. Amounts that reduce taxes for everyone is reasonable.
Grandpa Rhd, Dec 15 2005

       Maybe they should make relative velocity speed cameras too then. [+]
quantum_flux, Mar 14 2008


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