[GuB-42]

That's one thing I found weird driving in the US. Everyone consistently drove about 5 mph above the speed limit, which I ended up doing too as driving at the speed limit felt like being a nuisance, and was probably less safe too. I remember joking that American are so much into tipping that they even tip speed limits.

But why? If people, including law enforcement are comfortable with doing 70mph on 65mph roads, why not make the speed limit 70mph? Why is there an official and an unofficial speed limit? I heard even self driving cars are programmed to go at the "unofficial" speed limit.

For the context, I live in France. We have a lot of automatic speed traps that will systematically fine you for going 5 km/h above the speed limit, which isn't a wide margin. It means that either you are speeding, or you are driving at the posted speed limit, there is no "speed limit + tip".

[sgerenser]

I think the 5-10 mph above the limit that most people drive originates from the fact that police in most U.S. jurisdictions will almost never ticket someone going less than 10mph over the limit (except perhaps in school zones or construction zones). Due to that, the unofficial limit over time naturally drifts up to just under where you're likely to be ticketed.

This unofficial leeway likely developed due to things like mechanical issues with speedometers and tires causing reasonable doubt about actual speed within a few MPH. If they were to raise the official speed limit by 5-10MPH, then people would just do 5-10MPH above that. If police then started enforcing much more strictly, you're going to jam up the courts with more people contesting a 1mph over ticket as being due to speedometer calibration or whatever. Or just in general being much more resentful of the police for being so draconian.

[vv_]

> That's one thing I found weird driving in the US. Everyone consistently drove about 5 mph above the speed limit

This is the same in Europe as well. At least from my experience(-s) of driving in Lithuania, Netherlands, Poland and Germany.

> If people, including law enforcement are comfortable with doing 70mph on 65mph roads, why not make the speed limit 70mph

The speed limit is set based on what is considered safe upon collision in a particular area. Furthermore, if you increase the speed limit from 65 mph to 70 mph, you'll likely see drivers going at 75 mph.