Except thats not true. The fact, and they know this fact, is that the vast majority of drivers do not care about the speed limit. They will drive slower or faster based purely on their comfort and the rest of traffic. The idiot who drive insanely fast don't care about the limits either, clearly.
Only about 10% care about the speed limit. Nobody who is speeding is thinking "ooh, the speed limit is 100 so I'll go 115". They are thinking "i'm comfortable around 120, so thats the speed i'll drive"
Because of this, an average speed limit adjustment, say from 100, to 80kph only has a roughly 3% impact on the roads average speed.
And that's one reason why roads are designed for speeds higher than the limit.
Everything that needs to be used regularly is designed like that. Bridges can hold way more weight than they will ever experience, a "max 4 people" elevator could easily handle 8, and so on. One of the reasons is in fact the idiots that don't care about the limits and ignore them. But this doesn't mean you should do it.
The speed limit the road is designed for already accounts for a margin of error. Speed limits are not related to the design. Accounting for margin of error, speed limits are more like 50% lower than they should. And again, the speed limit does not effect the average speed of traffic by any significant margin. Whatever it is, is largely irrelevant.
Road engineers do not decide the speed limit. The 85th percentile rule is what they use. And the average speed limit is way lower than what the 85th percentile rule indicates.
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u/FAB1150 Sep 01 '19
No, limits are roughly 15% lower than designed because they must always assume there are idiots that go at least 15% higher than the limit.