Originally Posted by
mattotoole
Road engineers do not do this.
If speed limits are adjusted, they're set at the 85th percentile -- the speed below which 85 percent of the traffic is actually travelling. That's rare though. All roads have a design speed, and most are governed by statute, according to the type of road.
I know what is supposed to happen.
The 85th percentile rule for instance encompasses 24 hours a day and does not take into account all users of the road, such as cyclists. Also road design speeds may have nothing to do with the current speeds on that road... look at roads that were laid out 40 years ago for 30MPH traffic that now have speed limits of 50MPH. (these exist in my area... )
I also know of roads that have been built recently that are signed for 60MPH, as there was no development on the road, but since, the roadside has filled with homes, and businesses, and speeds have not been adjusted accordingly.
Also the fallacy of the 85 percentile rule is it assumes drivers are driving in a safe and reasonable manner while using the road.