Driverless Cars and Driverless Trains

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.

Design Evolutions

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.

Lawyer Up

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.

Update: Platooning

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.


16 thoughts on “Driverless Cars and Driverless Trains

  1. keaswaran

    I suppose one way to put the distinction is this – for cars, driverless technology allows closer following distances, which increases capacity at peak times, but does little or nothing for capacity at off-peak times; for trains, driverless technology allows lower operating costs, which increases capacity at off-peak times, but does little or nothing for capacity at peak times.

    The main advantage rail has is at peak times, because its capacity is so much higher there. But off-peak frequency is very important to allow people to travel one direction at peak hour without being stranded if they stay late or leave early.

    1. Alon Levy

      Well, driverless technology for trains does allow for more capacity at the peak, but mainly because it’s bundled with general signal modernization. SkyTrain’s minimum headway is 75 seconds, and I think the same is true of the automated lines in Paris (or is 85 there?). Usually subways are limited to 120, which is a lot higher than the stopping distance limit, and includes generous safety margins.

      With cars, those safety margins aren’t there. Freeway capacity is based on the principle that actual stoppages don’t happen much (hence sub-brick wall rule headways), and if they do, then shit happens.

      1. anonymouse

        SkyTrain’s capacity study indicates that the peak service that they think they can run is 39 trains per hour. Coincidentally, this is exactly the same as what Moscow thinks is the limit of capacity on their system, which doesn’t have CBTC or driverless operation. (There are drivers, and a fixed block system with automatic speed enforcement based on coded track circuits). The constraints on rail capacity come not just from signal headway but also station dwell time and to some extent train acceleration as well. Things like terminal layout can also make a big difference if you get it wrong.

      2. letsgola Post author

        I think LA Metro’s design headway for LRT is 100 seconds. But of course thanks to NFPA 130, I doubt they could hit that in the tunnels.

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  3. Joseph E

    Platooning will never work for driverless cars.
    Tires blow out. Engines fail. Deer jump out onto the freeway. Earthquakes happen. These events can all lead to one vehicle suddenly swerving into another lane or decelerating.
    When the liability is on the maker of the driverless car hardware and software, the manufacturer will need to design a headway sufficient for the cars to stop in a worse-case scenario, without causing a huge multi-vehicle pile-up.
    This is likely to REDUCE capacity compared to existing conditions on freeways and expressways. 2 second headways lead to accidents every day on motorways, and this will no longer be acceptable when the computers are in charge, and large multinational corporations or governments with deep pockets are the target of lawsuits.
    Driverless cars will be a great solution to lack of parking and high cost of taxis in cities, but they will worsen capacity constraints on major roads and especially on motorways.

    1. letsgola Post author

      Reduced capacity is certainly a possibility, though many people seem not to have entertained it. Certainly, the experience of the railroads with cab signalling was a loss of capacity, and PTC will likely reduce capacity as well (at least, in the way they’ve decided to implement it).

      My guess is that if it looks like driverless cars will reduce capacity, that outcome will be found unacceptable, and we’ll end up with human drivers with a lot automatic assistance. The rail analogy would be the MBTA Green Line, where nothing other than wayside signals has ever been installed, because no one’s ever figured out how to install cab signals or CBTC without reducing capacity.

      1. Alon Levy

        It’s strange that CBTC reduces capacity – the point of it is to allow infinitely fine-grained blocks, which should raise capacity. In New York it certainly is raising capacity, although not by much, except on the L, which has a bad terminal.

      2. letsgola Post author

        To be clear, in the vast majority of cases, CBTC for transit would increase capacity by allowing infinitely fine-grained blocks. On the railroad side, PTC may reduce capacity, because most railroads are just installing it as an overlay system that will enforce stops at the wayside signal locations. The stop target won’t be the train in front of you, it’ll be the red signal in front of you.

        The MBTA Green Line is just a unique wacky case for transit – nothing today is actually enforcing the stop. So any stop algorithm that’s more conservative than the human operators is going to reduce capacity. Think of the stations like Park St where they double-berth and often pull up to within a foot or two of the train in front – the CBTC would have to be pretty aggressive to allow a train to enter the station, even at 5-10 mph, and pull up that close. The signal blocks in the tunnel are so short that except where there are sharp curves, they’re really just supplementing line of sight operation. I think all anyone is hoping for on the Green Line is a CBTC system that increases safety without decreasing capacity, since safety is the real issue there.

  4. threestationsquare / Anon256

    Why does implementing driverless trains require hundreds of millions of dollars and years of disruptive construction to install CBTC systems, even in places like Paris with generally good transit-planning cultures? Why can’t the current job of the human driver be replaced by a computer with a camera and computer vision software to recognise the existing wayside signals and start or stop the train as they require? This would be far, far simpler than driverless car software, yet I’ve never heard of it being attempted anywhere in the world. Of course this doesn’t yield the capacity benefits of CBTC, but it vastly reduces labour costs for potentially very low capital cost.

    1. letsgola Post author

      For transit lines, the job of the human driver really already has been replaced by automatic train operation and cab signals. Sky Train in Vancouver is driverless; other lines (LACMTA Green Line and PATCO for example) were designed to be driverless, but they still have drivers, usually due to union rules (what else).

      So I think the people installing CBTC are really doing it to get the capacity.

      1. threestationsquare / Anon256

        I’m asking about systems like New York, which weren’t designed to be driverless but seem like they should be much easier to automate than cars. Retrofitting cab signals etc on such systems is very disruptive and expensive, but if driverless cars can obey stoplights then surely driverless trains can obey wayside signals.

        If it’s just union rules, then why aren’t systems in places with weaker/more reasonable unions automated? Every Tokyo Metro line still has drivers. Also why do union rules matter when you can just fire every single driver the next time the union contract is up?

      2. letsgola Post author

        I suppose that on a system like NYC, you could automate using similar technology to driverless cars. The technology would have to be good enough to berth a train, react if it sees a person fall off the platform into the track, etc. I’m not sure there would be any safety benefit in NYC, since stopping is already enforced by trip stops. There’d also be little capacity benefit, so you’d only be saving on labor.

        Theoretically, you can face the union off in a big strike (and in fact, many freight RRs along w/ SEPTA did this in the 80s after they were freed from restrictive regulations on crew requirements). But transit providers are public agencies, which means they’re political agencies. Politics in NYC is just not going to allow the MTA to fire all the union workers. Transit agency decision makers answer to politicians, and politicians answer to voters.

      3. threestationsquare / Anon256

        “Only” saving on labour, when labour is a significant majority of the MTA’s operating expenses, would be enough to make the subway operationally profitable, and beyond that allow massive increases in offpeak service.

        Not all transit providers are public agencies, for example the transit providers in Japan generally aren’t. Further, strikes are almost unheard of there. Why don’t they retrofit their trains with car-style driverless systems? There are plenty of other places in the world where the transit operators are relatively insulated from politics and labour action is unlikely, but I know of nowhere that has implemented driverless trains without expensive and disruptive signalling upgrades first. Why not?

      4. letsgola Post author

        Is driverless car technology really to the point where a rail agency would feel comfortable with it being responsible for safe operations?

  5. Ian Mitchell

    “safe braking distances for railroad signal design are calculated assuming no track brake is used.”

    This seems absurd to me. Why not design the operations around what the technology can offer?


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