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> When the speed limit is 65 mph, do they actually intend for everyone to go no faster? I don't think so.

Where is this supposed ambiguity coming from?

Some states follow Assumed Maximum Posted Speed (in certain places) and others are Absolute Maximum Posted Speed. It is not absolute in an Assumed Maximum Posted Speed state that driving faster than the posted speed is against the law and deserving of a fine, merely it is prima facie evidence that you were driving dangerously but can be challenged and overturned. For example, in Minnesota, outside of municipalities on highways (there may a few more qualifiers like posted speed is 55 mph or higher and might need to be a divided highway, I don't remember 100%) an officer can pull you over and issue you a ticket for merely driving faster than the posted speed limit. You can even admit you were driving faster (I don't recommend this). You can still challenge the ticket in court. If you can convince a judge that your speed was safe, the judge can let you off. If the weather is dry, temperature moderate, visibility great, no other people or vehicles around you, you were able to safely slow down, and (prima facie evidence) that you posed no risk as no one was injured by you driving faster. In Wisconsin though they are an Absolute Maximum Posted Speed state so if you are found to have been driving faster than the posted speed limit, that's enough to ensure you can be fined.

In Washington state, the State Patrol has gone on the record multiple times and with the media saying their enforcement is based on "Under 10 over", i.e. they won't pull you over for doing 69 or less on the Interstate in a 60 zone, or 79 or less in a 70 (assuming speed is the only issue).

Are you asking (1) why I think what I think?

Or are you asking (2) how we wound up in this situation as a society?

(1) I think what I think for several reasons. Basically everyone speeds. Probabilistically that includes he very lawmakers writing the laws, the police, and the road designers. I've also read some articles talking about road design, and in it it's mentioned that the designers factor in that most people will speed if the road conditions are amenable. I've also seen police cars driving around without their lights on, passing people at higher than the speed limit, and when unable to pass, the appear annoyed to me.

(2) I think this situation arose in sort of a "normalization of deviance" manner. Police didn't want to be too strict, or didn't want to bother fighting tickets for people speeding only a little, so only gave tickets for people speeding a lot. Then over time many people realized that, and started speeding a little. More are and more people started speed just to fit in with the surrounding traffic, until eventually everyone was speeding. Peer pressure. I've heard driving the same speed as the surrounding traffic is generally safer than driving significantly slower (or faster). Once everyone is speeding, that includes lawmakers, road designers, and police. And they factor that in when they write laws, design roads, and enforce laws.

There were also fairly contiguous moral panics in the late 1960s (teenage boomers crashing muscle cars) 1970s (fuel use) and early 1980s (slightly older boomers driving drunk) that lead to the regulations being written far in excess of what there's popular support to enforce which is a huge contributor to why the enforcers and judiciary are essentially responsible for dialing it back to something that doesn't make the system look stupid.

    "it's unclear what the lawmakers, road designers, and police intend"

In many cases, there's a gap between the original intention and the current need.

Many speed limits and policies were established in an era of fewer cars, but also much less capable cars with fewer safety features - many speed limits were established before the adoption of ABS, stability control and airbags, and more recent innovations in lane-keeping and adaptive cruise control.

Modern cars may be capable of travelling at greater speeds with greater safety, but there's a more recent recognition of the increase in emissions pollution from increased speed. Speed-limits typically remain grandfathered in at their original value (which may have been set 30, 40, 50 years ago), regardless of the change in context.

Then there are some pecularities such as the UK default of 60mph for a single-track road, but if you were to try that in many rural locations (think Cornwall, Scotland, Wales) you would likely find yourself upside-down in a ditch.

This post highlights the absurdity of some of the limits!

https://www.reddit.com/r/CasualUK/comments/1dng5z9/genuinely...

The UK NPCC (National Police Chief's Council) have a published policy where enforcement effectively starts at 10% +2mph over the speed-limit (whilst allowing officers to use individual discretion if they feel there are aggravating factors).

https://library.college.police.uk/docs/NPCC/Speed-enforcemen... [PDF]

Counterpoint: with mobile devices, and increasingly, control and information features of automobiles themselves, distracted driving is increasingly a concern.

There's also the point that driving capabilities vary wildly by individual, and often decline drastically with age. Recent case-in-point, an elderly driver in San Francisco who killed a family of four (a mother, father and two daughters, waiting at a bus stop, not in the roadway at all), let off with a minimal sentence, raising much public furore:

<https://www.sfchronicle.com/crime/article/sf-west-portal-cra...>

<https://www.sfchronicle.com/crime/article/san-francisco-west...>

The driver was speeding (70 mph in a residential area), and possibly driving the wrong way on the street.

> Speed-limits typically remain grandfathered in at their original value

That depends on where you are. In Texas, state highway speed limits are determined though a traffic study[1]. The monitor traffic for a while, then set the limit to the 85th percentile.

People can use this to get out of speeding tickets. If you find that it's been a long time since a speed study was done on the road you were on, the judge might throw the ticket out.

There are some hard limits though. For example, the maximum speed limit that can be set on a road is 85 mph.

[1]: https://www.txdot.gov/safety/driving-laws/speed-limits/speed...

In other words, 15% of people are ALWAYS speeding, by definition.

So with perfect enforcement (punish everyone over the threshold), how do you enable an 85th percentile rule?

> 15% of people are ALWAYS speeding, by definition

Not necessarily. If the data is heavily clustered around a single value, the percentile collapses to that number.

> UK default of 60mph for a single-track road, but if you were to try that in many rural locations (think Cornwall, Scotland, Wales) you would likely find yourself upside-down in a ditch.

The way the national limit is framed is more limit than road speed. It's interesting how we think of the limits as drivers: we get frustrated when other people go slower than the limit, we don't treat it as a limit, we treat it as the speed you should be traveling at.

I live fairly rural in New Zealand (UK expat) and even though you necessarily get a lot of speed variation on the roads around me, due to being winding, having farm traffic, sometimes narrow, you still get idiots who have to be going at the exact limit (or over) and tailgate 1m behind any vehicle in their way. Including trucks who can't really see them when they do that. I enjoy driving fast on those roads but I still don't understand the impatience.

The purpose of a system is what it does. In Australia, they want you to go 50. In the USA, they want a reason to fine you.

> In the USA, they want a reason to fine you.

In my area of the US, I don't think so. I'll go 9mph over the speed limit when I drive by a police car. I've never been pulled over.

In some other areas of the US, you're right, which is why I'm less likely to speed in other areas.