> Let's assume people are going at a speed that the road is designed to handle

Let's not, because most of the speeding I see is from people who very mistakenly think they know what speed the road's designed to handle, because they don't give a crap about pedestrians, bicyclists, visibility, other vehicles, or really, anything but getting to the next red light faster.

Perhaps you drive in a utopia that's solely populated by traffic architects, but here in the real world, it's populated by normal people, many of whom are bad drivers, and bad judges of road safety, but are really good at hitting the gas.

Our high death rates reflect this.