I doubt it. Both of these methods are very intrusive.

Mostly because I think it would glitch and get the speeds wrong

If it was 100% accurate and you couldn’t get a speeding ticket if it was active I’d be all for it

> prevented you from exceeding, say, 20% of the speed limit

I initially read this as "20% of the speed of light", and though you were being sarcastic.

Yeah, because need to be able to use your vehicle to escape pursuers and also as a ramming weapon. I assume police would get an exception from this rule, but they don't actually have more of a legal right to use their vehicle as a weapon than anyone else, they are just less likely to be questioned on their judgement of the situation as an emergency by the DA. Probably also a second amendment violation, but the Supreme Court might be too originalist (and not textualist enough) to buy that argument, as cars did not exist in the decades surrounding the founding.

Did you mean exceeding the speed limit by 20%?

Because what you actually said is true too, and hints at why "it would be safer" is not a good enough reason to implement something.

I'm fine with it notifying the driver it thinks you might be speeding, but I don't like the idea of actually limiting the speed of the car. I've used several cars which had a lot of cases where it didn't track the speed right. Driving near but not in a construction zone. Driving in an express lane which has a faster posted speed than the main highway. School zones. I've seen cars routinely get these things wrong. A few months ago I was on an 85 MPH highway and Google Maps suddenly thought I was on the 45 MPH feeder. Add 20%, that's 54 MPH max speed. So what, my car would have quickly enforced the 30 MPH drop and slam on the brakes to get it into compliance?

I'd greatly prefer just automatic enforcement of speeding laws rather than doing things to try and prevent people from speeding.

Honestly I would think something like transponders on every freeway would work better than GPS. Regardless, I think everyone in the thread could think of 10 technological ways of making this work. I think the biggest barriers are political, not logistical, and definitely not engineering.

So we spend a ton of money putting in transponders and readers in cars which still have various failure modes, or we just put cameras on the highways and intersections and say "car with tag 123123 went from gate 1 to gate 2 in x minutes, those are y miles apart, average speed had to be > speed limit, issue ticket to 123123".

The toll roads could trivially automatically enforce speed limits. They already precisely know when each car goes through each gantry, they know the distance between each gantry, so they know everyone's average speed.