On Maximum Speeds

One of the first things that made me interested in autonomous cars was the idea that in the future I’d be able to get somewhere faster than I can today.  I quickly came to the realization that this is from higher speeds and speed limits, less difficulty in “hard” traffic situations, and navigation around “trouble” spots.

This article stated that there are four approaches to setting speed limits.  The two prevalent in the US are Engineering approach and Expert Systesm approach (ie: using a computer program.)  It is reasonable to assume that a big part of what goes into a safe speed is the reaction time a driver has to an event, and the ability of a driver to keep a car on the road.  Both of these requirements (reaction time and vehicular control) computers can do far better than humans.

Since autonomous vehicles can operate with closer tolerences, several things will drive the maximum speeds on roads.

  • Road design – existing road design will affect safety
    • Many curvy roads cannot tolerate high speeds because the tires will “break traction” and the vehicle will leave the road bed (crash in the woods.)
    • Other roads have obstructions (curves and hills are the most common.) These obstructions, combined with possible overtake speeds will drive some speed limits.
  • Tolerable wear and tear on the road – If the road is a highway with truckers, those vehicles travelling at high speeds may tear up the road at an unacceptable rate if a reasonable speed limit is not set.
  • Vehicular maximum speed – If a car cannot go more than 120 MPH, the speed limit matters not.  The vehicle will obey physics laws over congressional laws every time.
  • Cohort/Fleet speed — even if your muscle car can go 200 MPH, if everyone else has a sub-compact limited to 100 MPH, it may not be safe to travel faster than the cohort.
  • Economical speed — Speed limits were set based on efficiency requirements in the US in the recent past.  This may occur again.
    • Individual cars may need to be throttled down to achieve a certain efficiency based on size
    • Cars may need to throttle down to allow drafting or bump drafting.
  • Event reaction time — It is currently hoped that if a deer or child or 18-wheeler retread enters your lane, sufficient time to brake is considered.  Note that this is not the case for high speed (or other) trains, and may not be considered for platoons in the future.

Currently, all lanes of a highway have the same speed limit.  I think it is possible that instead of speed limits, lanes have “speeds.”  You are either in the 90 MPH lane, or you are in the 140 MPH lane.  It may be that your onboard navigation system will permit you to go at a slower speed until you are approached from behind.  I think that if there are “platoon” speeds, many of the issues surrounding platooning will be overcome.  These issues include the difficulty of determining the difference in fuel efficiency, and how fast to go.  Additionally, it will help with what to do if your car is not sufficiently powerful to lead at a speed, but powerful enough to be able to draft.


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