It literally does not matter what the "safe speed" of a road is or how fast "free flowing traffic" will go. We have set a social contract, this is how fast traffic should go on this road. Pedestrians trying to cross the road now can accurately judge how quickly an oncoming car will reach them. Cars trying to pull out of side streets can now accurately judge how much time they have to enter the roadway. Cars entering the freeway can now judge how quickly the car behind them is approaching. It is impossible to tell if someone is coming towards you at 35mph or 55mph until it's too late.

It's not "criminalizing the conduct of 15 percent of motorists", those 15% of motorist choose to break the law and endanger everyone else. They're not genetically predisposed to one certain speed, they choose to go faster. And with any choice, it comes with a consequence.

But go ahead, say whatever makes you feel better about yourself. Hopefully the cop understands that your free flowing otherwise law-abiding speed is just poorly optimized. Hopefully the pedestrians you hit understand that their safety is just capricious and arbitrary. Hopefully the family of the victims understand you're just being unfairly criminalized.

Or you can just drive the speed that the entire rest of society has all agreed to drive. That's how civilization works.

> It’s a standard practice in traffic engineering to set limits based on 85th percentile speed of free flowing traffic in good conditions. That apparently tends to produce an optimal speed limit from a crash/mortality perspective, but has the side effect of criminalizing the conduct of 15 percent of motorists.

No, it doesn't, for several reasons, most notably:

(1) percentiles don't work that way; particularly, if everyone who would drive at or faster than the limit in its absence drove exactly at the limit in its presence, it would have no effect on the 85th percentile speed, so setting and maintaining an 85th percentile limit doesn't require any violations of the limit to occur,

(2) simple speed limit violations often are not criminal in and of themselves, so even if 15% were driving over the limit, they wouldn't be criminals.

Do you have a citation for that? It sounds plausible but I bet the real situation is more complicated.

The highway speed limit may be 70mph, say, but from an emissions point of view, for most cars the ideal speed is more like 60mph.

From a safety point of view, the ideal speed will depend largely on traffic conditions. 85mph might be fine in light traffic on a straight road in clear weather in daylight, less so in other conditions. That seems like a good argument for lowballing the speed limit (assuming that you're correct about that).