I’ve always been a fan of the idea of Personal Rapid Transit. For those who don’t know, PRT is a kind of driver-less taxi system on rails, somewhere halfway between a metro system and a private minicab.

People don’t like public transport, they don’t want to sit or stand in a confined space with a bunch of strangers, they like cars, the convenience, the personal space, and the freedom to go when and where they like.

PRT tries to capture the best of both worlds, the capacity benefits of public transport, with the joys of motoring, by providing a network of autonomous 2-4 person vehicles that can be switched through the network like the packets of a computer network, routing the individual from any source station to any destination station by the quickest route without having to stop or even slow down along the way.

The most famous PRT is the Ultra system, an implementation of which is being put in place at Heathrow Terminal 5 to take passengers between the business car park and the terminal building.

I recently came across another system being designed Aaron Patzer called Swift PRT. Aaron has put together an interesting proposal based on a set of limitations around weight, speed and cost to see if a suitable system is viable.

His initial results hit on some limitations around the amount of time and space vehicles require to slow down to corner or stop at a station, thus limiting the overall system capacity as other vehicles get held up behind them meaning vehicles have to be kept at a large distance from each other.

Another limitation Aaron mentions is the number of stations required to make a system that is faster than the car when the addition of the walk to and from the station is taken into account:

Personal Rapid Transit and Light Rail systems simply cannot put enough stations in even dense suburban areas to beat the automobiles.  While travel time once at a station and going to another station can be up to 2x faster than cars in light-rail, or 4x faster for Swift, this improvement is killed by walk time to and from stations.

If stations are spaced 2.6 km apart, this adds an average of 8.5 min of walk time to a station.  If the walk time is the same from a station, that’s 17 extra minutes per journey.  Let’s assume that your average driving speed is 72 km/hr (45 mph), and Swift provides a 3x speed advantage (216 km/hr or 135 mph).  For any distance less than 27km (17 miles, or 23 minutes of drive time) in one direction, it’s still faster to drive.  Even halving trip times is possible only in a handful of terrible traffic locations (Los Angeles, Washington DC and New York), where commutes average 45+ minutes each way.

My immediate thought on this of course is the addition of the bicycle to the equation to increase the range of the stations. It’s a well known fact that a bicycle can easily cover 4 times the distance that a pedestrian can in the same time, and so cut the number of stations required by a factor of 16.

This then leads to the question that if people can cycle to the transit station, is PRT really required or desirable at all? For journeys below 5 miles, the bicycle is personal rapid transit that already exists today. As mentioned in a recent Guardian article:

The big advantage of the bicycle is it exists, it’s not just a projected attempt to reduce emissions.

As for journeys over 5 miles into dense urban areas, the combination of bicycle and light rail is hard to beat (as proved by the Dutch and the number of Bromptons in London).

I like the idea of PRT, I’d love for it to become a reality, but as Aaron finishes up with, the future of PRT might actually be the self-driving car:

It is the ubiquity of roads, more than the greatness of cars that is difficult to defeat.  And so, the future of transportation is, perhaps disappointingly, simply better cars.  Our abstract criterion for the perfect transportation system is one that is fast, ubiquitous, has on-demand departure, and is quiet, private, and safe.  The solution to all of these is a self-driving car.