Mass transit remains the most viable method of transporting people over dense, multi-mile terrain.
The last mile and "normal" metros like Memphis, Baltimore, or Cleveland, where it is not dense enough to justify a comprehensive subway/light rail system, are probably the best use cases for self-driving cars and universal car sharing systems.
There is at least one lane on either side of the road at full capacity. Always bumper to bumper. But, the cars do not move save 2% of the time when their operators remove them from the traffic jam, I mean parking spot. There are 275,450 streetside parking spots in San Francisco city limits. Assuming a pooling of some drivers, and an increased utility of roads due to size & traffic flow then we can look forward to a net decrease of cars on roads and a net increase of available land for expanded sidewalks and bike paths.
Rail mass transit does not as efficiently use the land it's on a occupancy basis (there's not always a train on a specific square foot of track). In peak times, cars are more efficient on a vehicle basis. According to BART system facts[1], there are 107 miles of track. There are 669 cars, seating for 72 in 448 of them with each being 70 feet (with 59 of them having an additional 5 feet for a cab), and seating for 64 in 230 cars[2] (with an indeterminate car length, so I'll use the smaller listed), for a total capacity of 46,976 seated people. BART states that all cars can hold over 200 people in a "crush" load, so we'll assume 200 as the theoretical maximum, and say BART can carry 133,000 people when at peak (crush) capacity, and over the 107 miles of track, that gives us a density of 1,250 people per linear track mile, but with only 8.3% track utilization at any one time.
Cars do not as efficiently pack people per vehicle usually, but can more efficiently use the roads on a per-vehicle basis. Assuming very heavy traffic which is not stop-and-go, so perhaps 35 miles an hour average (the same as BART), and a 4-lane highway (two each direction), if each car is allowing two car lengths between itself and the car in front (slow traffic), we have approximately 33% road utilization (or 25%, or 20% depending on what you think the average space between vehicles is). Since carpooling seems to be at about 10% currently carpooling[4] (ignoring that it may be different in certain arterial routes, as we are discussing), we have around 1.066 people per car[5] as a lower bound. With an average car length of 177.2 inches[6], or 14.77 feet, we can estimate the people per mile on the highway during this time as being 604 people per mile of 4-lane highway.
Interesting take-aways for me:
While 4-lane highways may take more room than rail (not sure the actual sizes here), they are also more versatile.
If the highway bogs down below 35 mph, it's then less than the average rate of BART, and we need to start computing people over time instead of just people over distance.
BART has much more room to increase track utilization, but there is likely unaccounted for overhead here on each train. Optimal usage at current speed is one train arriving immediately after the prior one leaving, at 35 mph exact speed and 20 second stops, for a train of six cars (?) and 425 feet, that would be cars 2.42 train lengths apart, and a utilization of 29%, or roughly a 4x increase over current rates, 5,000 people per track-mile.
Cars have much more room to increase vehicle utilization. If we replaced 50% of vehicles with full size vans transporting on average 7 people each and didn't touch road utilization, we would be at an overall average of 4 people per car, and 2600 people per highway mile. Interestingly, if we somehow moved towards a system where smaller vehicles picked up and shuttled people with small amounts of sharing to bus-stations where they were sorted into smaller buses (40 people) going specific area depots, and from those depots dispersed again to final destinations using individual cars with small amounts of sharing, we might easily surpass rail transit systems. averaging 20.5 people per vehicle, but with somewhat more area used should put us close to 10,000 people per highway mile.
Of course, there's a lot of assumptions in all the numbers, and some speculation in the possibilities, but I thought it was interesting to figure out. ;)
1: http://www.bart.gov/about/history/facts
2: I know the total car numbers don't add up. Complain to BART, it's their data.
3: (66970 feet + 695 feet)/(7 * miles * 5280 feet/mile) = (47125 feet)/(564960 feet) = 8.3%
4: https://www.census.gov/prod/2011pubs/acs-15.pdf, table 1.
5: 105,476 drove alone, 13,917 carpooled, if we assume all carpooling was just two people per car, we get person to car density by (105,476+13,917/2)/105,476 = 1.066 people per car
6: https://www.reference.com/vehicles/average-length-car-2e8538...
7: 5280 feet/mile / 14.77 feet/car * 0.33 highway mile utilization = 117 cars at highway lane mile utilization. 117 cars * 1.066 people/car = 126 people per mile of highway lane. 4 lanes fives us 604 people per highway.
I can't really think of any reason why one technology is inherently higher-capacity than the other, it's mostly that private cars are very inefficient, not rubber tyres.
An autonomous car system that supports easy ride sharing, and semi-public transportation through something like uber for busses can fulfill much of the same need, but without quite the expected density, but with the ability to deliver to just about any location.
Both would benefit from a last-mile service . Sort of like arteries and capillaries in the circulatory system.
Actually, you know what would be really cool with this? Autonomous bikes[1]. Not that you need them to drive you, but that you can rent a bike and have it show up for you, or rent it at the bus depot/train station, and have it return after you've reached your destination.
Crossrail in London will have a train every 2.5 minutes with a capacity of 1500 people per train.
Trains also do not slow down to a crawl at high capacity.
