wumpus wrote:For highway traffic, this should be trivial to compute by determining the spacing of cars possible by a human, and the spacing of cars possible by a human.
For US highway traffic, assuming 70mph traffic (sorry about imperial units, but metric units are only used to repair cars in the USA):
4 seconds (what I needed to write on my driving test in the 1980s): ~400 ft
2 seconds (what is currently recommended) ~200 ft
1 second (typical in Maryland driving) ~100ft
0.1 second (possible for a computer?) -10ft?
A factor of 10 assumes that either the drivers aren't tailgating (which likely only happens in places that *don't* need more roads) or the computer simply has *zero* spacing between cars. A factor of 3 to 5 seems possible, although how you would be certain that a human didn't slip into your convoy is beyond me. Don't forget the aerodynamic advantages to all that drafting as well.
You would be certain that human drivers didn't slip in simply because they can't. At 10 foot spacing, no one can change lanes without being part of the computer network, and asking for space between car NY-ABC123 and car CA-4DEF567. A 15-foot-long car won't fit in a 10-foot hole.
I don't want to
ride at a distance where one dropped packet (I'm gonna stop: all cars in lockstep behind me, please hit the brakes now) means a giant pileup. Waiting for sensors to positively determine the car in front of you is braking implies nonzero reaction time and would require more space. Computer clock speeds might be in the GHz, but sampling rate on radar/sonar is in the KHz to (maybe) 10s-of-MHz range. And the brakes themselves are mechanical actuators with reaction times in the milliseconds, not micro- or nanoseconds.
Even with a theoretical zero reaction time, 10 feet isn't going to be reasonable. It is only safe if every car has exactly the same braking ability. But, even just looking at Ford (pulling numbers from Car & Driver online reviews), you have a range of 145ft to get from 70mph-to-0 for a GT, to 209ft for an F-450.
If the truck is following two GTs, each at 10 foot spacing, and they all brake (at max power with zero reaction time) because a deer jumped into the road, the truck drives right over the top of both
GTs and hits the deer in front of them: 64 more feet to stop, 20 feet of space, just over 31 feet of GT. (Yeah, it would really just push all 3 vehicles into a single pileup. Driving over the top like a mini-monster-truck sounds funnier, though.)
Even the difference between a plain-old Focus (179 feet) and the rally-car-trim Focus RS (154 feet) is way too much for a 10-foot following distance. Or a baseline Taurus (180 feet) vs. a baseline Mustang (156 feet).
I suppose every car could broadcast its expected braking distance, knowing its own brake and tire wear, road conditions, etc, (dynamically, to adjust for expected brake fade, even) to keep the following distances minimal, but they aren't going to be "10 feet" except in the most laboratory-esque circumstances.
Safe spacing = reaction time + difference in braking ability. 200 feet for a human is usually reasonable (at 70mph, that gives you 1sec reaction time plus 100 feet of braking delta, and humans are a bit faster than that if they're paying attention, and 100 feet is a generous delta; that's a moderately-worn big SUV following a new sports coupe).
So, an all-human 215-foot pitch between vehicles (200' space + 15'-long car) versus a very-generous 75-foot pitch (60' space + car) gets close to a 3x improvement. Something under 2x is a more-reasonable estimate, given tailgating in high-traffic areas cutting the human distance down. (115' to 85' is 1.35x)
(Then we can add the triple-trailer semi trucks to the equation... or even just F-150s with big camper trailers with brakes, or small U-Haul trailers without brakes...)