Autonomous car kills pedestrian: not the entire story
Urban design and vehicle type are as culpable as Uber
As inevitable as the headline is and as tragic as the death of 49-year old Elaine Herzberg is, this isn’t as simple a story as it initially sounds. It’s as much an indictment of car-centric urban planning as autonomous vehicles and while deeply significant socially, emotionally and to her family and friends, it’s statistically inevitable, especially where it happened.
Tempe is part of Phoenix, Arizona. This city has the distinction of having the third highest pedestrian fatalities in the USA per 100,000 residents. It’s a car-centric area with few amenities for pedestrians or cyclists.
The spot where the accident occurred is a stretch of road over a mile long with no pedestrian crossings at all. It has a bike lane along one side and Ms. Herzberg is reported to have been pushing a bicycle across the road when she was struck by the Uber. Any stretch of road that has elongated stretches with no crosswalks and people living, working and shopping on either side of it sees regular jaywalking simply because people on foot can’t be reasonably expected to go over a mile out of their way on foot to cross a bit more safely.
And ‘a bit more safely’ is the operative word. Suburban intersections such as the ones at either end of the stretch of road where Ms. Herzberg was killed are much less safe than urban intersections designed for pedestrians. They are wider, the corners are cut to allow cars to drive more quickly and speeds through the intersections are faster. This is well understood in urban planning and traffic safety. This study from Dhaka is just one of many which find the same results. Suburban intersections are considerably less safe than urban intersections. People being people, they know this. They also know that suburban intersections have lights optimized for automobile traffic flow. Not only would a pedestrian have to go far out of their way on foot to get to the intersection and back, they would also expect to wait minutes for the light to change and the conditions of crossing would not be appreciably safer than crossing in the middle of the suburban stretch of road.
There is no universe in which people living near this road won’t cross it far from intersections. They might walk to the intersection once, but they won’t do it twice.
The road is reported variously as either a 35 mph or 45 mph zone. According to Google Streetview, the speed limit is 45 mph in the direction that the accident occurred and 35 mph in the other direction. The police have reported that the speed limit at that point was 35 mph. The Uber was traveling at 38 mph at the time of the collision per the police.
What the data shows is that regardless of whether the speed limit was 35 or 45, the fatality likelihood shoots up over 30 mph. Inertia, the tendency of a mass to keep moving in the same direction, is a function of the square of velocity. That means that as the speed of a car increases, it hits a lot harder with a little bit more speed.
At the 38 mph the Uber was traveling, there was a median 40% likelihood of Ms. Herzberg would be killed, but an upper range of 60% likelihood. At the 35 mph speed limit, those numbers are still 25% and 40%. At 45 mph it’s 80% likely that you will die when hit by a car, with a potential of 95%.
This wasn’t helped by the type of vehicle, a Volvo XC90 SUV. Inertia, once again the force that kills pedestrians when cars hit them, is also a function of mass, and SUVs are heavier. The Volvo SUV is 2,020–2,160 kg (4,450–4,760 lb) depending on the spec. And as a study pointed out recently, all of the extra gear from autonomous capabilities adds mass as well. With a driver/observer in the car, it’s likely that the Volvo was over 2,260 kg or 5,000 lbs.
And SUV designs don’t give much latitude for pedestrians to do something besides be hit straight on by the equivalent of a fast-moving brick wall. Sedans with sloping hoods and windscreens may cause a lot of damage when they hit a person, and there’s still a significant chance of a fatality, but there’s also a better chance of the person rolling up the hood and windscreen and surviving. As the reference points, out, this isn’t new news. That’s a 2003 reference, and the studies haven’t change the outcomes since because physics is physics.
Due to these factors, pedestrians die daily in large numbers in the USA when hit by cars.
In 2015, 5,376 pedestrians were killed in traffic crashes in the United States.This averages to one crash-related pedestrian death every 1.6 hours.
Source: Centers for Disease Control and Prevention, Motor Vehicle Safety, https://www.cdc.gov/motorvehiclesafety/pedestrian_safety/index.html
There’s been an increase since 2015 with distracted drivers and pedestrians, with over 6,000 pedestrian fatalities in 2016 alone.
None of this is to take away from the tragic death of Ms. Herzberg or to diminish the need for more safety testing with autonomous vehicles. But it is to point out that the way cities are constructed, the speed limits that are set and the types of vehicles being driven lead to pedestrian fatalities regardless.
There’s increasing evidence that autonomous cars are part of the solution, although obviously not in the case of this Uber. Tesla is seeing a lot fewer collisions when their cars are doing the driving. They have stated and the National Highway Traffic Safety Administration has confirmed that after a couple of billion miles of their cars driving under semi-autonomous control, airbag deployments dropped by almost 40% compared to when their cars driven by humans. This is quantitatively better safety, and it’s going to get better from there.
Autonomous vehicles tend obey the speed limits better than humans, so where speed limits are designed with pedestrian fatalities in mind — not the case here — autonomous vehicles are less likely to be speeding and cause deaths. Autonomous vehicles don’t get distracted, an increasing problem, so their sensors will detect people where a human’s eyes might be on the car’s or their phone’s display. Autonomous vehicles react much more quickly than humans, so when a pedestrian is detected, braking will start much sooner and any impact will be much slower. And that’s compounded by the lower speed to begin with, as inertia makes a difference to how fast speed is scrubbed off in braking as well. 45 to 35 mph takes longer and further than 35 to 25 mph.
Autonomous vehicles are, reasonably, being held to much higher standards than human drivers are. But the failure of an autonomous vehicle to stop before killing Ms. Herzberg is part of the larger poor urban design choices that have been made and continue to be made to make drivers lives’ slightly better at the expense of life itself for other road users.