I think there's something off about your arguments, human drivers speed, but they don't do it because of properties of the speedometer or the cruise control, or air resistance, or potholes - they do it because they don't care much about the speed limit.
It's completely possible to stay below the speed limit in a car, a lot of people just don't bother. Cruise control won't apply the brakes (at least in older cars) if you encounter a significant downhill gradient - that's up to the driver, and while speedometers are often out and GPS is more accurate, speedos universally over-read in my experience. If you're doing GPS 38mph your speedo is probably reading 40, which is not really within an excusable error margin of 35mph IMHO.
I would expect a fully autonomous vehicle to be able to accurately obey the speed limit and I suspect the reason it wasn't/doesn't is because that behaviour has been programmed in or developed. You could make arguments that doing what the other cars around you are doing is safer than being an outlier, but that's not the same argument you're making. I think the 38mph figure is significant.
Current systems cannot know the exact speed of a car due to the mechanics involved:
A speedometer's reading is measured on the gearbox, at the output but there are a myriad of little things after that including variance in the final drive, tire size and tire pressure all of which can make the "calculated" speed and the real speed to be within a few % of each other. All of these components are also affected by wear too.
At the moment you need a GPS to conclusively measure a car's speed with >99% accuracy but GPS signal isn't available/sufficient everywhere. One could argue that we should create some sort of highly accurate local system that measures speed and we certainly can but we'd be trying to solve a 0~10% speed measurement problem: we'd get no real value from it.
In other words: a car measures how fast a specific cog is rotating, adds a few variables and estimates how fast the car is going based on that.
edit: and all of this assumes the car is going straight, calculating speed when the car is curving is more complex.
>they do it because they don't care much about the speed limit.
Oh, certainly, and I apologize if my argument came across as "It's not possible for people to not speed" or "People only speed because of the properties of speedometers or cruise control"
My point is that speed within a few mph is already inaccurate, and that 3mph is pretty much within the current margin of error.
>speedos universally over-read in my experience
I don't know that there's a common leaning one way or the other in actual variance due to speedometer construction, but over-reading is the most common occurrence due to tire pressure being low causing it, and a huge amount of people drive with low tire pressure. Overinflated tires, or moving to larger tires in general, will result in under-reads.
>I would expect a fully autonomous vehicle to be able to accurately obey the speed limit and I suspect the reason it wasn't/doesn't is because that behaviour has been programmed in or developed.
I don't agree. lagadu covers this plenty well in his comment, however.
It's completely possible to stay below the speed limit in a car, a lot of people just don't bother. Cruise control won't apply the brakes (at least in older cars) if you encounter a significant downhill gradient - that's up to the driver, and while speedometers are often out and GPS is more accurate, speedos universally over-read in my experience. If you're doing GPS 38mph your speedo is probably reading 40, which is not really within an excusable error margin of 35mph IMHO.
I would expect a fully autonomous vehicle to be able to accurately obey the speed limit and I suspect the reason it wasn't/doesn't is because that behaviour has been programmed in or developed. You could make arguments that doing what the other cars around you are doing is safer than being an outlier, but that's not the same argument you're making. I think the 38mph figure is significant.