[m463]

I drove a tesla with "full self driving (supervised)" and with the default settings, the car will only go up to the speed limit. You would think this should be ok, but there are plenty of 25 mph zones where even conservative drivers will do 30 or 35 mph.

I had to take over the driving in these places because cars would pile up behind me, drivers would get impatient, and in some cases people would get angry and tailgate or try to pass where they shouldn't.

Turns out there is a FSD setting that lets the car adjust to conditions, and it will "go with the flow", even going over the speed limit, but matching the traffic and harmonizing.

Note there is a concept traffic engineers use called "the 85th percentile rule" that helps design highways by setting the speed limit for maximum compliance. Using this rule leads to the safest roads.

Some folks seem to be outraged that people don't comply with the speed limits, but when they are set artificially low, speed variance increases and roads become more dangerous. I think driving the speed limit or below in these places might amplify this.

data on speed variance vs safety (fig 4):

https://www.fhwa.dot.gov/publications/research/safety/17098/...

[crote]

> Note there is a concept traffic engineers use called "the 85th percentile rule" that helps design highways by setting the speed limit for maximum compliance. Using this rule leads to the safest roads.

The fundamental flaw in this concept is that it completely ignores that 1) the ideal speed depends on external circumstances and intended road use, and that 2) road design heavily influences driving speed. If you want to have a safe road network, you have to design it for a specific speed. When done properly, drivers will naturally be driving at the intended speed. Speeding doesn't happen because it doesn't feel safe to speed.

Highways have to be straight and wide, without any level crossings. People are supposed to drive at high speeds, so it is designed in such a way that it is safe to do so, and it feels safe to do so. Local access roads are narrow, twisty, have relatively poor visibility, and are equipped with things like speed bumps. You're supposed to drive at slow speeds because there are lots of level crossings and driveways, so they are intentionally designed to make high-speed driving unsafe to the point of being physically impossible.

[m463]

Maybe my wording is confusing, but the idea is not flawed. It is used to set the speed limit, given the road.

If you want a lower speed limit, you can just set it, and if below the 85th percentile speed, you'll get a less safe road.

The proper way to do it would be to adjust the characteristics of the road so that drivers slow down and the speed variance is minimized.

Same for increasing the limit. A wider or smoother or straighter road may work.