Comment by Animats
7 days ago
Freeway capacity is maximized around 35 MPH. Faster, and the greater distance between cars reduces capacity. Slower, and there are not enough cars per minute per lane. So the goal of ramp metering signals is to throttle input to keep the freeway speed around 35 MPH.
I think you would want to keep the road just slightly less dense (fewer cars, higher speeds) than the density that maximizes throughput, because otherwise you operate at the edge of an instability. Any tiny local deviation in speed somewhere triggers a slight local decrease in throughput, causing bunching which further decreases throughput and snowballs into a traffic jam.
When operating at slightly faster than max capacity, local slowdowns cause a local increase in throughput, allowing bunches to dissipate.
Doesn't really matter - in the real world everybody is tailgating. Drivers need to maintain 3 seconds (GP was using 2 seconds which safety experts have not declared too little) between cars for safety reasons. However drivers are instead maintaining more like .5 seconds between cars. As such nearly every city needs about 5 times as many lanes as they currently have when people finally start demanding "just one more lane"! 5 times - that puts Huston level freeways in Des Moines.
If you maintain the proper following distance at maximum capacity when there is an issue you can momentarily drop to 1 second (while braking) and then expand back to normal and there is no effect on traffic.
I'm going to double-down on this point because it does matter. In the real world some people are tailgating, but that does not cause traffic jams if and only if the road is running below peak capacity, where negative feedback self-corrects those traffic density deviations and gradually smooths everything back out. (Unless the tailgater causes a crash, of course).
But if the average following distance is such that the road is exactly at the peak of throughput, or any smaller, then any momentary dip into tighter following distances pushes the road it into a positive feedback operating mode, which triggers a traffic jam.
If you haven't seen it, here is a classic experiment demonstrating the effect: https://www.youtube.com/watch?v=Suugn-p5C1M
(But obviously the solution to traffic isn't 5 more lanes, it's viable alternatives to driving!)
This is a classic case of throughput v. latency - and most people are going to prefer lower latency, i.e. driving faster.
Interesting - I have believed for many years that it was around 17 MPH. I felt that this tallied with my observations - as traffic levels increase, vehicles slow down (increasing total capacity) until it falls to a critical speed (when slowing down reduces capacity) and then it changes to stop/go.
In my experience (on UK roads) this critical speed is around 17 MPH - but it might be a little different elsewhere.
Freeway capacity chokes at the limit giving rise to basically overpacked lanes slumping from metastability; systemic control with ramp meters or I66 (within the Washington D.C. Metro area) style real-time dynamic pricing lets freeways flow properly around peak capacity if properly implemented.
There are roads that regularly suffer from acutely insufficient capacity in many metro areas; specifically, repeatedly at times _the dynamic pricing toll that would discourage enough people from using it to stay uncongested_ would overshadow the price of a rental-with-driver (Uber-style) during off-peak times. It's not that the people shouldn't get through; it's that most people won't need more than a backpack worth of luggage with them and could thus be packed 3~4 passengers for each driver. Splitting the toll would be the reason to do so.
Unfortunately only really dynamic congestion tolls would really fix the concept of rush hour traffic jams. And the necessary surveillance system would bring severe mass surveillance/tracking concerns with it at least in central Europe.
I don’t think the surveillance needs to be cameras, a combination of radars (for velocity) and induction loops or the axle-counting rubber hoses (for counting) should do the trick
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35? That seems too slow. Several years ago some pranksters in Chicago drove side by side at 55MPH and caused a MASSIVE backup for miles.
That's more of an illustration that throughput isn't something people want than an illustration that throughput is higher at 75 than at 35.
You see a lot of blanket assumption, in discussions of traffic, that throughput should be maximized, and almost no examination of whether increasing throughput is a goal that makes any sense.
For example, working from home has catastrophic effects on throughput.
It’s throughput in combination with demand. That’s why Chicago has reversible lanes on the major highways. Inbound and outbound throughput needs vary during the day.