Speeding is one of the most common ways for Americans to break the law. Drive the speed limit on the highway around here and you'll typically be the slowest car on the road. How much over the speed limit is customary varies regionally, but drivers often expect cops to ignore them at 5-15 mph over.

Overall, I think this is a pretty bad situation. It gets people used to ignoring laws, people who scrupulously follow the law are often at higher risk (and cause higher risk to those around them) than if they went along with traffic, driverless cars go awkwardly slow, some risk of selective enforcement, confusing for travelers, etc. How can we get out of this?

If we just started strictly enforcing the current limits we'd have a mess: it's too big a behavior change to push all at once so you'd see even more dangerous variance in speeds than today, and it's unclear we actually want people driving the posted speeds. It also wouldn't work well to raise the limit to the speed people are mostly going, since many people would assume they can then go an extra 5-15mph on top of that.

Instead we could take inspiration from Brazil and introduce a parallel system of maximum speeds:

Initially this has no legal effect, and just makes the existing amount of leeway more legible. On a 55mph road where people normally drive 60-65 and the police don't start ticketing until you're more than 10mph over, the signs would say both "speed limit 55" and "max 65". These would be rolled out gradually, in consultation with traffic engineers and the people responsible for enforcement.

As they roll out, you adjust enforcement to match. Put up speed cameras set to the maximum in many places, and in other places have police enforce the max strictly after each sign is put up. Traveling above the limit but below the maximum becomes effectively allowed, since there's no enforcement.

Once the rollout is complete you overhaul the laws around speeding to make the maximum the legal limit, and adjust rules that are set relative to the old limit to still make sense. For example, if you previously gave only low fines for going 58 in a 55 zone, and in practice never issued them, while you gave high fines for going 68, you would still want the higher fine for going 68 in a "max 65" zone. The goal is to bring the law in line with behavior, but otherwise keep the status quo.

At this point you could consider removing the older lower "speed limit" signs, but I think it's probably worth keeping them as advice about what speed to travel. In some cases you might raise them a bit, knowing that with the maximum in place as a firm limit you'll get slightly faster speeds but lower variance.

I think there's a path here that brings the law back in line with driver and enforcement behavior, while otherwise essentially maintaining the status quo. It does require new signs and some policy tweaks, but seems on balance pretty positive to me.

New Comment
28 comments, sorted by Click to highlight new comments since:

I'm a bit concerned about the long-term effects of this plan (especially after laws formally change to disregard the old 'speed limit' signs entirely but strictly enforce the 'max' ones).

I believe that many current speed limits are much slower than the correct speed to drive at, and I don't think this is particularly controversial.  (I have driven on a perfectly straight, almost-empty six-lane freeway with a posted speed limit of 55).

This did not happen by random chance!  Aliens did not land and subtract 10 from every speed limit sign!  Existing pressures - revenue pressures from tickets?  political pressure from people who don't like cars driving fast by their house? - lead to speed limits being set too low.

If you switch to this new approach, in the absence of some reason to expect that not to happen again, I'd imagine that once your 'max' signs are the limit they will also be set too low.

I think US speed limits are so low in part because of an effort to limit demand for oil in 1973; see Wikipedia.

Even setting aside that ignominious origin, I expect that optimal speeds have significantly increased as technology has improved and so speed limits are probably too low because of inertia.


This assumes speed limits were correctly calibrated at some point. I think the actual cost of road deaths (which are arguably the top single cause of QALY loss even at current historically low rates) is high enough that I suspect it was originally set way too high and is still unreasonably high given the costs.

In the UK there is a non-binding but generally observed rule that speed cameras allow you to drive 10% + 2mph above the speed limit(e.g. 35mph in a 30mph zone) before they activate.

This is a bit more of a fudge but better than nothing.

If we were to replace speed limit signs, it might be better to go all out and install variable speed limit signs. It's common to see people failing to adjust their speed sufficiently in poor conditions. A few days ago, there was a 35-vehicle pileup with two fatalities in California due to fog.

The New Jersey Turnpike actually does have variable speed limit signs.

It gets people used to ignoring laws, 


I wonder if this is a good thing though? The greatest horrors ever committed in the world have been done by people following the laws and orders governments set out for them.  Having an obviously insane law that people experience on a daily basis might encourage some critical thinking about which laws are reasonable and which are not. 

Hard to test one way or the other but I still here people saying "It's against the law" as a self-evident reason not to do something far more often than I am comfortable with. 

People are not ignoring laws, they’re recognizing the limits of precise measurement, and understanding that the enforcement of the law is part of the crafting of the law. People do drive much more slowly in a 25mph than in a 55mph zone, and they’re punished differently for driving 90 on each.

It’s important to realize that the limits are set with the knowledge that it’s very hard to reliably (aka: court-upheld) measure small variations, so legislatures EXPECT it to be more of a nudge than a strict bright line.

But we, as a society, are annoying and will abuse a system that acknowledges this, so it doesn’t get written down that way, it just evolves into an equilibrium that most people just expect and live with.

they’re recognizing the limits of precise measurement

I don't think this explains such a big discrepancy between the nominal speed limits and the speeds people actually drive at. And I don't think that discrepancy is inevitable; to me it seems like a quirk of the USA (and presumably some other countries, but not all). Where I live, we get 2km/h, 3km/h, or 3% leeway depending on the type of camera and the speed limit. Speeding still happens, of course, but our equilibrium is very different from the one described here; basically we take the speed limits literally, and know that we're risking a fine and demerit points on our licence if we choose to ignore them.

Yeah.  Other folks have already mentioned that the degree of enforcement leeway in the U.S. increased when the federal government made artifically-lower speed limits a requirement of federal highway funding in the 1970s.  Which I can’t confirm or refute, but does make sense: I imagine that some states who disagreed with the change might have grudgingly set the formal limits in line with the federal policy, and then simply used lax enforcement to allow the speeds that they preferred all along.  I have noticed that it’s often seemed politically unpalatable for officials to stick to a program of stricter enforcement to rein in a particular area’s entrenched driving culture after speed limits were increased in the 1990s, though.

In any case, if folks think that part of the reason for lax enforcement is measurement error then that could be used as an input toward designing a separate maximum speed designation.  One could keep the “speed limit” enforceably defined in terms of the actual vehicle speed, while defining a new parallel “maximum speed” constraint strictly in terms of a measurement taken by law enforcement equipment that passes a particular calibration standard within a particular window of time before and after issuing the citation.  Then you’d end up with one standard that gives the benefit of doubt on measurement error to the driver and another that gives the benefit of doubt to the enforcement record, and thus there’s a logical reason for (at least some of) the spread between those two thresholds.  (This legal system might also make it easier to move toward maximum-speed enforcement that works more like existing license-plate-based tolling systems, allowing for a much more pervasive enforcement regime to push the culture toward compliance without the downsides of setting up lots of direct conflicts between irate drivers and law enforcement officers.)


I cannot recall having seen any recently but do recall seeing minimum speed limits posted on highways in the USA. That is sort of in line with your suggestion. 

I do think there are a number of aspect that matter with both setting speed limits, enforcing speed limits and driver's actually driving at some given speed. The two main aspects for me would be coordination among the drivers and congestion on the road. Both will have some connection to driver reaction time and vehicle capability (can the car physically change coarse, stop or accelerate assuming the driver has time to react).

Most of that is really situation dependent and even the speed range approach or some set max speed is a poor metric for defining some optimal rule. I sometimes wonder if fewer accidents might occur if outside certain areas (school/hospital zones, residential streets where lots of kids might be playing and the like) no speed was posted. Just have better liability assignment to those not driving at reasonable speeds. (Problematic from a practical perspective but not impossible given "blackbox" recorders and GPS data in cars and simple force analysis of the car(s) involved in any accidents.)  A concern here might be that even if we see a reduced frequency of events (seems to be somewhat supported by cases where towns/cities removed stop lights and people started viewing the intersections as if they were 4 way stops -- i.e., no one just assumed they had some right of way over the others.) A potential reasons this might not be a good idea would be if frequency declines but the when things go wrong they go very wrong -- never have fender benders but cars are totaled and people always crippled or killed would not be an improvement.

But I do think driving habits related to speed seem to be driven more by what would be a common law type process than the statutory "posted speed" law.

I cannot recall having seen any recently but do recall seeing minimum speed limits posted on highways in the USA.

Could you be thinking of Canadian speed limit signs, which say "maximum"?


I wasn't but have seen those as well.

The signs I was thinking of were those that stated Minimum 45 MPH on US highways. I think that also went along with the general rule that if you were driving under 45 on the highway you should have your warning flasher lights active.

Sorry, I actually just misread your post as saying "maximum" where you wrote "minimum".

Probably not, since some U.S. states do post minimum (fair-weather) speeds on Interstate highways.  Section 2.2 of this paper includes a slightly dated map indicating the minimum speeds in each state (where applicable).

I actually just completely misread my parents post and thought they wrote "maximum" and not "minimum".

Note that in many countries, the rules or fines may be different for cars and trucks/busses.

It gets people used to ignoring laws

Wishful thinking. Breaking speed limits does not in fact gradually turn people into anarchists: most people continue to obey (and worse, respect) the laws that are actually enforced. 

Based on the title, I thought you were going to go another direction. But isn't it insane that a typical consumer car is capable of driving significantly over 100mph? In large parts of the country, there's nowhere that it's legal to drive anywhere near that fast, and an ordinary driver will never have a legitimate reason to drive that speed. I understand why this hasn't happened, but wouldn't it also be better if normal cars just weren't capable of going over, say, 90mph?

In large parts of the country, there's nowhere that it's legal to drive anywhere near that fast

Nowhere legal on public roads; you could take your car to a track.

wouldn't it also be better if normal cars just weren't capable of going over, say, 90mph?

I mean, probably, but isn't people driving 100mph+ a tiny fraction of deaths from speeding?

If you make a car with a max speed of 65mph by decreasing the amount of force available, it will be:

  1. impossible to pass cars safely, because you won't be able to overtake quickly
  2. very difficult to maneuver while going near 65 mph, because you won't be able to accelerate quickly
  3. very annoying to change lanes, because while you are vectored to the side, you will be going less than 65 miles per hour down the road because you will max out at (65 * sin(theta)) mph, making it difficult to speed up while changing lanes, which is often considered good form
  4. very difficult to go up steep hills at speed, because you will use most of your power fighting gravity.

It's very difficult to decrease max speed by decreasing performance without creating a car that is much worse.

The thought would be that it would be the same car, but with some kind of software/hardware limit that prevents it from continuing to speed up once it reached some set speed, like 85 or 90. Not to limit the power train.

Where I live, I don't see many people going 15 over. I see most people going within -3 to +9 of the speed limit. They're following the law -- maybe not strictly as written, but as socially understood. There are a few people breaking the law -- and they get ticketed, etc. There are places where the speed limit is unreasonably low, and gets ignored (e.g. speed limit drops from 75 to 55 for construction zone, but no construction activity is visible), but in general, people around here follow the speed limit -- as socially understood.

The social definition is that +9 is OK, and it is logically based. Speed limits are only given to the nearest 5 MPH, so arguably (not legally arguably, but in people's heads) going 4 MPH over is not really speeding. Then, you need some kind of reasonable cushion. It defies common sense (but not the law) that a certain speed is perfectly legal in a variety of reasonable conditions, but 1 MPH faster than that is a crime. So, you have to have reasonable padding, say one 5 MPH increment. Thus, if you're going within 9 MPH of the speed limit, you're not speeding, just bending the speed limit, pushing into that safety margin.

Bumping a speed limit from the current value of 55 (+ social tolerance) to 65 (no tolerance) would not work. It would quickly become 65 (+ social tolerance), because you have not addressed the arbitrary nature of the speed limit. With strict enforcement, you could reduce the social tolerance value (I believe it was smaller before the 1970s national speed limit of 55 fostered widespread disrespect for speed limits), but it would difficult to eliminate it.

I doubt that speed limits are helpful at all. The sections of the German Autobahn with no speed limit (roughly 70%) have half the mortality rate per distance traveled of American highways[1]. Granted, the average American driver is probably worse than the average German Autobahn driver but hey.

How about instead of doing some random proposed change with speed limit maximums and what not we do some AB testing and figure out what's safer?

Of course safety concerns don't exist in a vacuum. Every second we save on the highway by going fast is another second of life we get to spend doing something actually enjoyable.

  1. https://en.wikipedia.org/wiki/Autobahn#Safety:_international_comparison ↩︎

German Autobahns with no speed limit have been designed to be safely driven at high speed. For example, wide lanes, long straight sections, very large radius of curvature for non-straight sections, minimal layout changes, good drainage. And also features which minimise the impact if accidents do occur, e.g. strong central barriers.

It does not therefore follow that removing speed limits on typical American freeways, which have not been designed for high speeds, is a sensible thing to do.

Plus, the way US politics works, if you did any kind of no speed limit trial, it would not last long. Let's say you're a politician who somehow gets approval to push through a policy to trial no speed limit on a freeway. An accident happens (regardless of whether speed was a cause), and you'll be out of office the next day, and that's the end of the trial.

How about instead of doing some random proposed change with speed limit maximums and what not we do some AB testing and figure out what's safer?

Rolling out this proposal on some randomly selected matched pairs of high-fatality roads and comparing outcomes would be relatively cheap.

One of the problems here is that our measurements are not precise (both in-car measurements the driver can see, and even more so for the outside law enforcement measurements). Especially when we take into account that cars don't have to be on cruise control, and often are not on cruise control.

That's why it is also assumed that if one is close to the speed limit, there is more room for doubt.

Fines, actually, do tend to increase gradually in many US jurisdictions, but insurance points (which often tend to cost way more than fines) tend to have sharp step-wise increases (and plea-bargains are done around those insurance point thresholds, reflecting the measurement uncertainty once again). Your MAX does roughly correspond to the first increase from the minimal number of insurance points for speeding to the next level of insurance points.

I think that you are correct, policies that "everyone knows" aren't "real" tend to reduce the degree to which everyone takes other policies seriously.  But I think a lot of the "unreal" policies are in place for reasons of liability, risk management, or other communication tool.  Also, seldom are any policy actually absolute or meant to be absolute.  Just ask your lawyer, nearly everything in life is negotiable.

What's more, speed limit policy is geared towards a complex set of goals, politically decided upon in a risk-managed, engineered way.  Then voted on by a board of people who might somewhat understand the problem.

You know what is almost never discussed explicitly in politics.  "What are our goals here?" and "What tradeoffs do we suspect these different decisions entail?"  Making this explicit, and letting people vote on politicians based on all this lucidity would be great, but reading, say, Thomas Schelling, I think it is also utterly impossible.

So we bluff speed limits (and nearly everything), and negotiate about it later.