Updated with a note on platooning and some input from R Winston Kappesser (@ronaldkappesser).
There was some back and forth on Twitter today on the potential of driverless cars and their impact on rail infrastructure like transit and high speed rail. In that context, here’s a civil engineering perspective on the technological issues and potential impacts.
Stuck to You Like Rubber on Asphalt, or Steel on Steel, or Something
First, we have to understand the technological differences between rubber-tired vehicles and steel-wheeled ones. That starts where the rubber hits the road or where the steel hits the steel.
When it comes to transportation, friction is both our friend and our enemy. We need some friction; otherwise, when you hit the gas your tires would just spin in place, or your rail wheels would just do something like this. Friction between the rubber tires and the road’s asphalt or concrete surface is what keeps cars and buses from flying off the road at corners. It’s what turns the tractive effort of a big honkin’ locomotive into forward motion.
On the other hand, too much friction wears out your car’s tires and makes your car run less efficiently. For trains, friction management is a critical part of track and vehicle maintenance. If there’s too much friction, the rails and wheels will wear out faster, and the train will use more fuel, increasing maintenance and operating costs. Friction management is so important for railroads that locomotives are equipped with sanders, so that the engineer can drop sand on the rails to increase friction on upgrades, while sharp curves are equipped with greasers to reduce friction between the wheels and rails. There’s an entire sub-industry built around friction management.
In general, the coefficient of rolling resistance between rubber and asphalt is about an order of magnitude larger than that between steel and steel. This means there’s proportionally more rolling resistance between your car and the road than there is between an Expo Line train and the rails. The very low rolling resistance on railroads is part of why trains are so much more efficient at long-haul freight than trucks. The coefficient of friction is also lower for steel on steel than rubber on asphalt.
Can’t Stop, Won’t Stop
That efficiency comes with a cost, though, in braking performance. Trains can’t brake as fast as rubber-tired vehicles. How much worse? For a 70 mph design speed, Caltrans Highway Design Manual requires a stopping sight distance of 750 feet. For a 70 mph design speed in territory with cab signals, the standards used by Amtrak and many commuter railroads require a safe braking distance of 4,942 feet. For high-speed trains, the stopping distances for purposes of rail signal design can be in excess of 2 miles.
(Note: some vehicles, notably LRT vehicles and some high-speed trains, have electromagnetic track brakes that use electromagnets to “grab” the track, allowing the vehicle to stop much more quickly. These brakes are used for emergency only; safe braking distances for railroad signal design are calculated assuming no track brake is used.)
The other major difference between rubber-tired vehicles and steel-wheeled ones in this regard is the ability to steer. A person operating a rubber-tired vehicle has the ability to take evasive action to steer the vehicle away from a hazard, while a train engineer is obviously helpless to do anything other than brake.
These technological realities have resulted in a different evolutions of civil engineering design standards.
For cars, design is predicated on the driver being able to see further than the distance needed to stop the car. The design of vertical curves (changes between upgrades and downgrades) is governed by the need to ensure the ability of the driver to see over the top of the hill, or for the car’s headlights to illuminate enough of the road ahead of a sag curve. At horizontal curves, vegetation and other obstructions on side of the roadway must be cleared far enough back from the edge of the road to allow the driver to see around the curve. In a safe design for autos, the driver will always be able to see further than needed to stop the car.
In contrast, with the exception of low-speed streetcars, for trains it is simply impractical to design the track such that the engineer would always be able to see further than the distance needed to stop the train. Horizontal and vertical geometry of the track is controlled by vehicle performance and passenger comfort. Safety is ensured by the signal system providing the safe operating speed to the engineer, and in some cases enforcing that speed, based on the locations of other trains (or perhaps more accurately, based on information that sections of track ahead of the train are not already occupied by other trains).
Note the fundamental difference here. For cars, safety is based on the ability of the driver to passively gather information about conditions on the road. For trains, safety is based on active collection of information on the locations of trains, and active dissemination of instructions to trains that it is safe to proceed.
Driverless Technologies, and Others
This means that the interfaces and impacts of driverless technologies will be different for cars and trains. For cars, passive decentralized technologies (i.e. the car just gathers information, and doesn’t communicate with other cars or with a central control center) will suffice. For trains, centralized control is a necessity.
For cars, it will be a huge improvement for safety simply for driverless cars to more reliably and consistently do the things that we currently rely on human drivers to do. This will have some positive impact on practical capacity by reducing accidents. If driverless car technology allows cars to follow each other more closely than they do today, by eliminating the component of following distance related to human reaction time, that will increase road capacity.
For example, you may have already figured out that, despite the stopping sight distance being 750 feet at 70 mph, cars on a freeway flowing at 70 mph don’t actually space themselves 750 feet apart. At that rate, a freeway lane would only move about 500 cars per hour, but the actual capacity of a freeway lane is about 2,200 cars per hour. If you have a driver’s license, you may (hopefully) remember the “two second rule”, that you should leave about 2 seconds of travel distance between yourself and the car in front of you. At 70 mph, that’s a little over 200 feet – less than the stopping distance, and acceptable only because you can see further than just the car in front of you, and you have time to swerve out of the way if needed. Part of that 2 seconds is an allowance for human reaction time; if driverless cars allow that component to be eliminated, they will increase capacity.
On the other hand, safe design for trains is based on maintaining at least the stopping distance between following trains. At 70 mph, a train should never be less than 4,942 feet behind the train in front of it. In practice, the distance will always be larger due to the impact of grades and the use of fixed signal blocks. The engineer’s reaction time is portion of that stopping distance, but it’s not much. Driverless train technology has been around for decades, but the primary appeal is reducing labor costs, not increasing capacity.
If the goal is to increase capacity on rail transit, communications-based train control (CBTC) will probably offer more benefit than driverless technology, because it will eliminate the capacity waste caused by fixed signal blocks. CBTC should also allow railroads to take advantage of better braking performance available in newer rolling stock. The combination of CBTC and driverless trains would allow many transit systems to greatly improve service by increasing capacity and reducing labor costs, thereby allowing the agency to provide more service.
A big unanswered question, in my humble opinion, is the liability implications of driverless vehicle technology.
For cars, what will be the standard for safe following distance? At present, we allow drivers to follow each other at less than safe stopping distance. Will driverless cars follow the “two second rule” or will they be allowed to follow more closely? If there’s a rear-end collision, who is liable? Note that some of this must have been decided implicitly or explicitly by the people who have operational driverless cars, like Google.
For trains, at present, railroad signaling is based on the premise that the train in front of you is at a stop, and therefore you must be able to stop too. If you implement CBTC you could argue that if you know the position and the speed of the train in front of you (Heisenberg be damned), you should be allowed to follow more closely. On the other hand, if the train in front derails for some reason, it’s going to come to a stop very quickly, and any following trains that are less than the safe braking distance behind are hopelessly screwed. There’s not a consultant in business in the country today that’s going to sign off on allowing trains to follow each other at less than the safe braking distance, and I doubt any agencies would do it either.
Therefore, for trains, I really think the capacity improvements are going to come from CBTC, not driverless technology.
The Future is Uncertain
I hope this post doesn’t sound like it’s down on transit. For one thing, any improvements available to cars will be available to buses as well. And nothing is going to change the simple geometric advantages that transit enjoys in dense areas.
Predicting the future is hard. If you’re out there predicting the doom of the car every year like James Howard Kunstler or the dominance of self-driving cars in 2020 like Randal O’Toole, you’re probably going to end up looking foolish.
A conservative approach would be to continue investing in cost-effective transit improvements, including CBTC and driverless technologies, where warranted. Automated car technology should, at the very least, result in a considerable drop in the number of people killed and injured by cars, and for that alone, it should be welcomed.
A quick note on platooning, which is the idea of having driverless cars follow each other very closely, perhaps only inches apart. This would greatly increase road capacity, but it would absolutely require vehicle to vehicle, and perhaps vehicle to central control, communication, as opposed to passive information collection systems currently being used by the likes of Google. I think that getting such a system operate reliably and safely will be more difficult in practice than many people expect. I don’t think we’ll be seeing it any time soon.