Could self-driving cars solve rush hours?

by tribsantos

People on twitter were arguing that cities should not plan for when self-driving cars arrive because they would not solve peak demand. I tried arguing that even if they do not solve peak demand there might still be a bunch of reasons why cities would like to plan ahead for a transformative technology. But I think that they could very well solve peak demand. It all depends on what numbers you think are reasonable. Here are mine (which I think are conservative).

The most inefficient scenario is if each person has rides in their own self-driving car. Conveniently for me, this is probably the easiest case to calculate too. Supposing each person will go to work in their own self-driving car, cars could be very small. Congestion prices based on the amount of space occupied (here’s a planning issue already) would create proper incentives. I’ll say 1m x 1m, which is plenty of space for a person to sit down and be comfortable. Engine could be below them, technology above.

If there are only self-driving cars, even if they were not be attached to one another, they could still be much, much closer then cars today. I say 50 cm to the sides and 2 m cm from the car ahead/behind. With this I am thinking that they are usually only 1 m apart, but that space must often be made for lane shifts.

How many cars can fit the streets of New York this way? The total length of the streets is 508.38 miles or 820 km. If there was just one lane, there could be 820,000/3 = 273,000 cars in the streets of New York. What is the average width of roads? New York avenues are pretty wide , but streets much less so. There’s a lot of parking space which, if there were a switch to self-driving cars only, could go away, so even a narrow street could have three lanes. So I will say that there is, on average, three lanes in the streets and avenues of New York. Since each lane is 12′ (3.7 meters) , we have an average street width of 11.1 meters, where one can fit 7 cars with 1 m width, 50 cm apart.

So we have 273,000 * 7 = 1,911,000. That’s how many of these cars, at these distances, fit in the streets of New York at any given moment. For how long would cars be on the streets for a person to get to work? I am assuming an average commute of 5 km. I am also assuming that these cars would be able to travel at an average speed of 15km/h (~10mph). This could sound high to you in light of crossings, but I don’t think it should, since one of the great advantages of self-driving cars would be to rationalize crossings, with every car moving at exactly the time that the light goes green. Since these will be electric cars, which accelerate pretty fast, they would most of the time travel at the average speed of 30 km/h (~19 mph). If they spend one minute at a traffic light for every minute they are driving, then they travel at 15 km/h.

Traveling 5 km at 15 km/h means you are there in 20 minutes. So, on average, every twenty minutes, 1,911,000 will cross the streets of New York and get to their destination, opening up space for other cars. In one hour, we would have 3*1,911,000 = 5,733,000 people reaching their destination, on a commute which would take only 20 minutes on average, door to door.

How good are these numbers? They are pretty good. They are actually extremely close to the total daily ridership of the New York subway system, which, according to Wikipedia, was 5,597,551 in 2014. In the worst case scenario for self-driving cars, we would assume that all of these trips happen at peak demand hours, and there are only two hours which are peak during a day – one in the morning, one at night. Even then, self-driving cars would be twice as effective as the busiest subway system in the West.

But capacity would probably be even larger. Congestion charges would create incentives for people to fit in the smallest space possible. I would just get my phone and say where I wanted to go and some app would show five other people very close to me who also wanted to go somewhere very close to where I was going to. If it was a concert or a game, whole buses could be easily filled with people going to close places.

If this is going to happen, however, there are some key actions that governments will have to take – most importantly, charge adequate and proportional prices for street use. So it is something that local governments most definitely should be talking about.

 

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