Friday, May 15, 2015

The problems with autonomous vehicles in cities

When I hear about how autonomous vehicles are the panacea for urban traffic woes, I've been skeptical. With every apparent advance—like the recently reported hand signal patent—I come back to the fact that they apparently still don't work in the rain (it hasn't rained much in Mountain View recently) and certainly not snow. And they've driven 700,000 miles, but mostly on the same subset of suburban roads that have been meticulously mapped, not new and varied roads across a variety of terrain and regions.

But I see the main issue is that autonomous vehicles will be unable to interact with people in the same way that people driving cars can. There's a turn in Central Square in Cambridge from Mass Ave to Pearl Street (I happen to live a few blocks away) that's a good example. It is right by a subway entrance and major bus stop, so there are tons of pedestrians. It has a couple of crosswalks but no signals. To make the turn, you often have to wait until pedestrians "screen" oncoming traffic, worm your way left, and find a gap in the pedestrian traffic to slowly get on to Pearl Street. Everyone is okay with this if it's done at a slow speed—if you waited until conditions were perfect, you'd never make the turn. But by the letter of the law, none of this is legal. To make the left, you would have to wait until there is no oncoming traffic (rare), whether there are people in the crosswalk or not. Then to cross the crosswalk, you have to wait until there are no pedestrians within 10 feet of the crosswalk, a situation which might occur here once every 45 minutes. (It should be noted that crosswalk laws vary slightly from state to state, so rules would have to be coded differently for each jurisdiction.)

People driving cars can get through this intersection. But an autonomous car can't be coded to break laws. If (more likely, when) it was involved in an accident, the code would be subject to discovery, and examined at length. If anything was found that permitted the car to break a traffic law, it would be legal ammunition to go after the deep-pocketed developers. In cities, everyone fudges the law a bit for things to work. Pedestrians step off near-if-not-in the crosswalk. Cyclists nudge over the line and leave a light a few seconds early. Cars slowly make turns when they don't have right of way and pedestrians break stride so the cars can get through. It's not perfect, but works. Sort of.

So, take a look at the picture to the right. In the middle, note that there is a person waiting at a crosswalk. An autonomous vehicle would see this person and stop to let them cross the street. But they would just stand there. The person might flick their hand, but the car won't easily be able to interpret that as "oh, I'm waiting, go ahead." Which puts the autonomous vehicle—or its coders—in a conundrum. If you stop for a pedestrian but they don't cross, how long do you wait? 5 seconds? 10 seconds? A minute? Can you really have any rule there that would allow you to break the letter of the law and cross a crosswalk with a waiting pedestrian? And how often will an autonomous vehicle come to a complete stop because a person is near a crosswalk, even if they aren't crossing? Will there have to be a manual override? However will that work?

Yet everyone driving a conventional car was able to quickly and easily tell that this woman was not crossing the street. Every so often she would wave her arm. An autonomous vehicle would have little ability to discern this movement. Waving at a friend? Waving at a person? Waving cars ahead? Waving because she was just released from an underfunded mental institution and waves at everyone? It turns out, the waves were pretty well correlated with the passage of taxicabs. Aha, she was hailing a taxi! This is something that every driver was able to intuit immediately, and no one stopped for her at the crosswalk. But an autonomous vehicle would be stuck. For how long? Who knows. Imagine a cab traversing 5th Avenue in Midtown Manhattan? It would wind up screeching to a halt every block as a New Yorker flung out a hand to try to nab an empty cab.

Sure, there might be fixes to this specific problem. Maybe Uber will render taxis a thing of the past. (But then won't people waiting on the curb for an Uber create the same type of problem?) Maybe every taxi will be fitted with a transponder so that an autonomous car can see a person waving and correlate it to the location of a taxicab and make the connection that they're hailing a cab. (Which seems to be a complex solution to a simple problem.) Maybe people will learn to hail cabs only away from crosswalks (fat chance). And even if this problem is somehow solved, there are thousands of others like it. Driving a car in a busy city has infinitely more scenarios than on a controlled access highway. You can control for one outcome, but there are thousands of others that may pop up.

And many of these will be one-off scenarios. The aforementioned intersection at Pearl Street is probably unlike most any other in the world. Cyclists are not required by law to use hand signals. A car may not signal, but may shade to the right, something some motorists (and many cyclists) know means they're probably going to turn. To get around a double-parked car, you may have to cross the double-yellow line. Human drivers can pretty easily assess the scenario and risks involved and decide whether to operate outside the letter of the law. But an autonomous vehicle, in any of these situations, may find itself stuck. The whole selling point of the self-driving car is that it will be able to operate without a human driver. But in cities, it seems that there would have to be frequent manual overrides when the car finds itself between a rock and a hard place: where it can't go forwards without breaking a rule.

People are good at breaking rules—and in city driving, bending the rules is frequently a necessity. Computers—by design—are not.


  1. And this is one reason why I have a rather different vision of the deployment of autonomous vehicle technology. I think it will be much more common, and eventually mandatory, on highways, where the rules are relatively clear and the speeds are high, and where the limits of human reaction time are most problematic. Most of the technology for full autonomous driving on highways already exists on some higher end cars, and it's only a matter of time until it filters down to a large fraction of new cars. Meanwhile, having the option of high speed automated operation would eventually allow limits on the speed of manually-driven cars, so no more 50 mph speeders in 25 mph zones.

    On a related note, I also think that automation technology has much more immediate promise for things like streetcars: they're confined to a track, and they generally have the right of way, and when they don't, there are signals, so a lot of problems get much simpler. And automated streetcars have the potential to change the transit landscape pretty dramatically.

  2. Automated vehicles work decently when there are fixed and limited guideways. They'll do relatively well on limited access highways, but need to get there. As for streetcars, the issue is that when they operate in mixed traffic, they're at the whim of other traffic. Vancouver has a lovely light metro system that is fully grade separated and fully automated. But it's not cheap. And the cheapest way to build is above ground, but no one likes elevateds.

    1. Well yes, obviously any competent city (such as Houston) will build most of their streetcar network in physically separated lanes. The interesting question is whether automation technology is good enough to move from 100% grade-separated to a more light-rail like setting, with grade crossings, or running in a street reservation, or even dedicated lanes. It seems to me like a much easier problem with many fewer variables than a fully autonomous car in urban traffic, though it still requires a fair amount of new technology.

  3. It seems odd to assume that the people writing the software for autonomous cars, most of whom will themselves posess driver's licenses, will only be aware of the laws and not the real world challenges.

    Elon Musk has talked about how slow speed (10 MPH?) operation will be safe with the sensors Tesla has been building into the Model S vehicles manufactured in the last 10 months or so, because the ultrasonic sensors will be able to detect objects within a range that the car can stop.

    He mentioned bicycles as a major challenge for autopilot, and I suspect that the underlying point here may be more that vehicular cycling near high speed automobile traffic is not as safe as its proponents would like to believe.

  4. I would like to thank you for the efforts you have made in writing this article. I am hoping the same best work from you in the future as well. Thanks...

  5. At first when the autonomous car hits the market, I was happy with the advancement of technology. In order to avoid accidents, technicians have done anything possible but alas, such problems have raised now. I could not believe that there something mistakes happening in such vehicles but reading your blog made me get into the depth and understand the things. I too feel like there are some missing codes in it. Well, hope for the best for autonomous cars! Hybrid Maintenance Dania.

  6. I too agree that there are few demerits of autonomous cars. Recently Google's autonomous car had accident but still the best thing to happen. Not only Google but some other auto developers are their trying to deliver the first error free autonomous car. We cannot ignore the fault there but still we should desire for an autonomous car because the demerits are just few but lots of merits are there.

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