This is such interesting stuff. If you go just one step further and plot the carrying capacity of a road against the speed of traffic on it you get a hump which tends towards zero as speed reaches infinity.
The significance of that hump is that it represents a maximum carrying capacity for a road - an optimum speed. If you do the numbers I believe it's about 50mph. What I think is really cool is that when the road is completely at-capacity, traffic is stable above 50mph (as cars can just slow down and capacity increases) but collapses to a jam below below 50mph (because as the traffic slows down to absorb new cars, capacity falls still further).
This is way why, when the M4 motorway funnels down to only two lanes as you approach London, there is a 50mph speed limit - it literally allows the road to carry more vehicles per hour. I also suspect that it's another benefit (other than safety) of setting 50mph speed limits in contra-flows and during roadworks - it allows the road to carry more vehicles despite having fewer lanes.
The significance of that hump is that it represents a maximum carrying capacity for a road - an optimum speed. If you do the numbers I believe it's about 50mph. What I think is really cool is that when the road is completely at-capacity, traffic is stable above 50mph (as cars can just slow down and capacity increases) but collapses to a jam below below 50mph (because as the traffic slows down to absorb new cars, capacity falls still further).
This is way why, when the M4 motorway funnels down to only two lanes as you approach London, there is a 50mph speed limit - it literally allows the road to carry more vehicles per hour. I also suspect that it's another benefit (other than safety) of setting 50mph speed limits in contra-flows and during roadworks - it allows the road to carry more vehicles despite having fewer lanes.