Collision energy and thus damage increases with the square of speed (or ~speed^4 for head-on) so there is still an interest in controlling speed.
Most drivers (especially those over 35yo) will auto-regulate their speed to the optimal (safety vs throughput) for the road design. The problem is the ones who don't. Speed limits are set lower than this optimal speed, partly to make it easier to stop and charge drivers that can't auto-regulate well. Most of the time you will be ignored for going 5-10mph over. If you are over that, it is seen as deliberate defiance and "you are asking to be pulled over".
Automatic enforcement turns this de facto road law on it's head however.
During the 1970's oil crisis, highway speeds were capped at 55 mph nationwide. It took several decades for this to reverse and only after safety studies showed that differential speeds (those obeying and those going the optimal natural speed for the road) is a significant contributing factor in crashes. Unfortunately, speeds limits are still often below optimal because of an assumption that every driver will always go at least 5mph over the limit (which is incorrect).
On 70 mph interstates away from urban/commuter traffic (where time pressures often affect driving), It's not unusual to see some cars going 5mph below the limit. That is a sign that these Interstate segments have the optimal natural speed.