I think that's stupid for one reason. As an engineer I often considered 10% tolerance to be sufficient. I think enforcing speed limits to anything greater than 10% is just silly. I often drift +/- more than 10% of my intended speed. It's just human nature. I pay more attention to potential dangers and obstacles, and less on whether I am within a % of the speed limit, and I really hope other drivers do as well.
Ignoring whether it’s “human nature” to vary by that much, the remedy is simple: in a 60mph limit (the clue is in the name), target 54. +/-10% is now 48.6-59.4, instead of 54-66. It’s within the speed limit, and a smaller absolute speed differential between lowest and highest speed.
Similar arguments have been made against average speed cameras on UK motorways - “I’ll have to be entirely focused on the speedo the whole time!”. No, you just drive within the speed limit.
Interestingly, you'll fail your UK motorcycle test if you don't largely treat the speed limit as a target.
If you stick at 40mph or thereabouts going into a 50mph zone, the examiner may think you haven't seen the speed limit change. More importantly, if you don't keep up a good pace on a clear country road with good visibility where the speed limit is 60mph, they won't have confidence in your riding at higher speeds, and they know - as all bikers know - that you will almost certainly spend a good chunk of your riding career at higher speeds on country roads.
They do have to check whether you are able to confidently, safely and within the bounds of traffic-rules, operate the vehicle close to the speed limits. This need not be taken to mean that one has to always ride close to the limit.
Of course, given that on two wheels, one is not as safe as on four (and also being enclosed in a metal frame). So, naturally the good riders will find ways to ride safely and that includes riding at appropriate speeds. The ones who aren't good/safe riders are not likely to ride for long.
The reasoning I see is that objects moving at the same speed in the same direction tend not to collide.
I have seen advice to motorcyclists that they should be just a bit faster than most traffic. Slow enough to manage changes in traffic ahead, but fast enough that no one is approaching you from behind.
Platitudes like this and the evergreen "speed kills" can themselves be dangerous.
Obviously driving at the speed limit all the time is not a legal requirement and there will be times when driving slower is appropriate. However, driving far slower than the speed limit for no good reason when there is other traffic around can itself increase the risk of something bad happening, because you won't be keeping up with the flow of traffic and behaving as other drivers will expect.
My suggestion for someone having difficulty keeping their speed in a 20% window was to aim 6 mph slower at highway speeds. That doesn’t constitute “far slower”.
In the Uk, if you are incapable of handling your car on a highway with other vehicles driving 12mph slower than the limit, then you are incapable of driving on highways - some vehicles e.g. HGVs are restricted to these lower speeds.
An unfortunate extension of this is that sometimes, a speed limit will be set very low for no apparent reason, and almost everyone will exceed it significantly if it's not visibly being enforced. This puts a driver who wants to be both safe and legal in a no-win situation.
Personally, I liked the idea put forward by one of the UK driver advocacy groups a long time ago. Speed enforcement measures should prioritise places with a dangerous hazard, and there should be prominent warning signs on approach in a standard format that show (a) the speed camera sign, (b) the current speed limit and (c) the nature of the hazard, so drivers understand why the limit is there and enforcement is visible and clearly justified.
Somehow, I suspect that if drivers had more trust that speed limits were being set appropriately and expected that they would actually be enforced where they most matter, they would become largely self-policing anyway. Then the actual road police could spend more time dealing with other problems like using a phone behind the wheel or drinking and driving, which are potentially more dangerous than most speeding but much more difficult to enforce without human intervention.
Risk compensation [0] is a relevant idea here. When you give people a highway they will treat it like one. If you want them to slow down, you have to make a road that feels slower. Tighter corners, narrower lanes, parked cars, bulb-outs, etc.
In suburban development especially, there is/was an initiative to make roads safer by increasing the physical tolerances relative to the posted limits. So you have all these residential neighborhoods with big gentle corners in them that any drunk idiot in a shitbox could easily handle at 60mph. And that is exactly what happens.
If 30mph is too fast for a pedestrian encounter, then 30mph needs to feel too fast for maintaining control / avoiding obstacles.
Designing road systems to encourage an appropriate driving style is valuable. Some non-traditional arrangements have been used to very good effect here in the UK in recent years, though personally I'd say the experiments have had mixed success overall and we've had our share of well-intentioned changes for the worse as well.
That said, it's also important to use these design techniques for clear reasons. Some people don't like motor vehicles and want to deter their use for reasons other than safety. They have been known to argue for road designs that limit capacity or force traffic to go very slowly as a deterrent measure, when perhaps a road designed to support faster speeds could have been just as safe and allowed a more efficient transport system. Sometimes, they even argue for measures that would likely be counterproductive given their stated goals, possibly without understanding why that would be the result. So before you can design a road, you first need to establish what your requirements are and what you're trying to achieve, which is more of a political question than an engineering one.
In particularly walkable cities you can see the extreme end of this in the way barriers are removed from between driving and walking parts of the road, and the driving parts are made narrower. Drivers take much greater caution and are forced to pay attention. Contrast to a wide asphalt road with sidewalks on the side, drivers feel separated from foot traffic and thus safe in their speeds and lose focus, even though they likely couldn't stop in time for a hazard even if they were paying attention.
Many American highways actually increase the length and size of the road markings in order to make speed feel slower than it actually is, which was likely a comfort thing at first but clearly it hides the present danger of speed.
I have never been on a busy motorway with everyone driving at or above the speed limit. Car drivers always vary between 55 and 70(+), and heavier vehicles are frequently as slow as 50. That is not a hazard. It would be a hazard were they to drive in the wrong lane, or fail to merge, but that’s a totally separate issue to the steady-state-avoid-speed-cameras which I was replying to.
This tends to be my rule of thumb. Speed Limit +10% assuming good conditions. It keeps me from being a hazard when other drivers wish to go faster while pretty closely adhering to the whole point of speed limits. 7 mph over the limit on the freeway is still pretty safe. 7 mph over on a narrow residential street can be quite dangerous.
This rule of thumb means I really don't have to waste any mental energy keeping an eye out for police, since someone else is almost always going faster and I find it doesn't significantly impact my trip times since traffic and stop lights tend to cancel out most potential gains from driving any faster.
Speed limiters and cruise control are pretty common in modern cars. You can set your mental intended speed lower and won't drift as much. This is from experience of switching to a car with speed notification beeper (very low tech) - it took maybe 2 weeks to stop drifting over almost completely without paying extra attention. We ended up treating it as a game with gf - shortest time for a 300km drive with no beeps.
I have a modern car. 2015 Kia Optima, and it has basic cruise control. I don't know what conditions you're driving in, but I think it's fucking scary to drive with it on unless there's a completely open highway in front of me. Not controlling my own following distance feels like I'm about to hit the car in front of me at any given moment, especially on a road where the speed limit is 75mph and you have people going anything from 60-90mph.
Never heard of a speed limiter. As you can probably tell from the "freedom units," I'm from the US if that matters.
I pop the cruise control on lots, especially on motorways. If I get too close To car in front I’ll touch the break, but even in the U.K. roads are rarely that variable - especially near the speed limit.
The cruise control has a switch to change to limiter, I rarely use that.
Adaptive CC - yes, pretty much all the time. I understand it would be super annoying to use basic CC in the city. But that's where you can set the limiter instead (they often come together with CC).
