You’re driving down a fairly major road with very few minor intersecting roads. You come to a set of red lights and (hopefully) stop. The lights are red for about 60 seconds. Now, if you look in your rear vision mirror you might see that only about 3 or 4 cars have banked up behind you in that 60 second period, yet you would be quite certain that many times that many cars would pass a point on the road in the same period.
So how come? Why do so many less cars seem to build up at traffic lights than would pass a point on the road in the same period? Anyone else noticed this?
WAG. Traffic flows in waves when there are traffic lights around. You think there is a lot of traffic because you are in with a bunch of other cars, but there is a lull in traffic after your bunch. The bunching is caused by the series of traffic lights backing the cars up then releasing them. So basically your perception of the amount of traffic is skewed and the 3 or 4 cars that are at the lights with you for 60 seconds is a better indication of average traffic flow.
First, you might be suffering from confirmation bias.
Second, cars approaching intersections w/red lights slow down in advance so as to delay the time they get to the red light (if indeed you are NOT suffering from confirmation bias and car flow is less, as you feel you have observed).
Generally, traffic patterns are things for which sophisticated mathematical formulas have been created, which attempt to use traffic control devices and other means to handle 80% of the traffic issues, and the other 20% cannot be managed so specifically because of randomness and crazy peak demands, so people are left to wait imperfect am’t s of time and it will seem ‘silly’ at times to sit at lights and such.
They only way to keep 100% of the traffic moving well is to over-engineer the roads and the control devices to a point where a 4-lane road would have to be 20 lanes to solve all problems at all times of the day for all days in a year. Toss in accidents and you might need 40-lane roads everywhere with billion-dollar traffic control devices with artificial intelligence.
Is your arrival and subsequent observation a random one? Could it be that some previous intersection has an effect on when you and others arrive, and distorts your perception, since you are assuming randomness?
I recommend a really fun and timeless book by George Gamow, Puzzle-Math, which has a story much like this. A railroad buff goes to a railroad track on the hour and on the half-hour, and counts the number of trains that pass in the next 15 minutes. He tallies the west- and east-bound trains, and discovers that there are twice as many going west as east.
So they are piling up in San Francisco, right? More likely, the train schedule isn’t evenly distributed around the hour. Perhaps more westbound trains pass during his observation period, and eastbound ones when he isn’t watching. He was unaware that the situation wasn’t random.
There are a number of reasons cars tend to travel in clumps. The first is simple statistics. If you drop 100 cars at random over a 10 mile road, there are a small number of possible distributions where they are all roughly 1/10th of a mile apart, but a very large number where there are one or more concentrations. Drivers tend to travel at different speeds, so on a single lane road the slower drivers will dictate the pace. Finally, features such as traffic lights and ordinary junctions also concentrate traffic together.
