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Tiny Vehicles and the New Safety in Numbers

New York City Council Member Robert Cornegy test drove a Bird scooter in New York recently. Photo: Gersh Kuntzman

With dockless bike share systems and electric scooters rolling into cities across America at an unprecedented rate, a holistic reduction in crashes, fatalities, and injuries should follow. Peter Jacobsen, author of the seminal “safety in numbers” theory of traffic safety, with his brother Joel Jacobsen, looks at how masses of tiny vehicles work towards Vision Zero.

On any weekend night in Washington, D.C.'s trendy Adams-Morgan neighborhood, you can spot — amidst the laughter and loud music — crowds of people traveling between blocks of restaurants and bars. You might notice some heads gliding along the wide sidewalks at twice the pace of others. Those folks are riding the electric scooters that, in a remarkably short time, have become ubiquitous in many urban neighborhoods around the country. First, we had tiny houses. Now, we have tiny vehicles.

The first generation of shared public bicycles were rented from a sidewalk rack, a system familiar from airport luggage carts. Newer, dockless shared bicycles instead use a smartphone app to direct the user to the nearest bike, which can then be left anywhere for the next user to find. Battery-powered scooters, grown-up versions of the children's ride with a small electric motor installed, rely on a similar phone-managed dockless system.

Two decades ago, the very idea of sharing vehicles with strangers would have struck most Americans as odd, even outlandish. Twelve years had to pass after Zipcar was founded in 2000 before even 3% of Americans had tried driving a shared car. But then, shared cars aren't visually distinctive. The installation of bicycle docks, by contrast, provided a constant visual reminder of the possibility of a shared ride, getting urban-dwellers used to be the idea. The dockless versions trumpet their convenience everywhere they're left. Americans' growing comfort with shared rides is reflected in the uptake rate for electric scooters, which flew past the 3% mark in less than 12 months.

It's not hard to understand scooters' appeal. Compared to other urban transport options, they're cheap, typically requiring just a dollar to start. They're convenient, needing no parking spaces. And, because they add little to congestion and produce zero tailpipe emissions, they can be ridden with a clean green conscience.

But what does the proliferation of tiny vehicles mean for traffic safety? Do they bring us closer to achieving the goal of Vision Zero, or are they just a faddish sideshow of no real consequence?

The answer begins by considering falling off one of the many new tiny vehicles on the street.

Any physical movement involves some risk of falling, of course. A recent New York Times article interviewed emergency room doctors, who are treating increasing numbers of scooter injuries, ranging in severity from skinned knees to head trauma. The San Francisco Department of Public Health is inaugurating a plan to collect injury data.

While we currently lack figures about scooters in particular, long experience has shown us that in an urban setting, walkers and bicyclists are unlikely to be injured fatally – unless they're hit by a motor vehicle, when the likelihood of serious injury instantly zooms. It seems reasonable to expect similar effects with scooters. Falling from a scooter rolling along at its top speed of 15 mph is doubtless painful, but nothing at all like being hit by a 4,000 pound hunk of metal hurtling at 30 mph or more.

However, a narrow retrospective focus on injuries incurred overlooks the possibility of injuries prevented. Strong empirical evidence suggests that the best thing we can do to ensure the safety of scooter riders is to increase their number. We have no shortage of data about pedestrians and bicyclists, and there's every reason to expect the experience of tiny urban vehicles will follow the pattern. The risk to walkers and bicyclists of being hit by a car decreases rapidly where more people walk and bike. Simply put, there is safety in numbers. Where three times as many people walk and bicycle, the risk to walkers or bikers halves. Even better than that, cities that encourage more bicycling with infrastructure changes experience a decrease in the raw number of bicycle injuries, and cities that implement bike share programs also see a reduction in the number of bicycling injuries. The five major cities that introduced bike share had a 28 percent reduction in injuries.

These findings, which may at first seem counter-intuitive, are a product of human perception. Our brains, which evolved over the long millennia before cars were invented, are too efficient for our own good. We scan our landscapes, as our hunter-gatherer ancestors did, for the common hazards, not the rare ones. There is little reward in devoting mental energy to a search for things we're highly unlikely to see, and so our visual cortex generally doesn't. Consequently, in cities where bicycles and pedestrians are rare, drivers are slow to perceive them. And at automotive speeds, "slow to perceive" is almost the same as not perceiving.

Moreover, this "low-prevalence effect" is unrelated to size. In places where motorcycles, and even buses, are rare, drivers find it disturbingly hard to perceive them. As pedestrians, bicycles, motorcycles, and buses become more common – as they move into the category of "known hazards to be scanned for" – the risk of collision steadily sinks.

There's no reason to believe the low-prevalence effect will exempt electric scooters and other tiny vehicles. But by the same token, there's likewise no reason to believe they won't benefit from safety in numbers. As tiny vehicles become more common, motorists will increasingly be on the lookout for them. More rapid motorist perception will translate into fewer collisions.

Not only that, but as the number of urban scooters increases, collisions will likely become less severe. In places where pedestrians and bicyclists are common, motorists’ collisions are not only rarer but less likely to be lethal, presumably because motorists slow down, reducing the kinetic force involved in the collisions that do occur.

The proliferation of tiny vehicles also has the potential to encourage more people to leave their cars at home, at least for short trips. Motorist deaths vary on a nearly one-to-one basis with motor vehicle use. The fewer car trips, the fewer driver deaths. If drivers can be persuaded to hop on scooters for everyday errands, we'll be that much closer to Vision Zero.

Over time, street design will likely adjust to better accommodate tiny vehicles. Their small size might give them a political advantage over bicyclists, who require relatively more space and have had to fight long years for the precious inches of asphalt cities grudgingly dedicate to bike lanes. Although it's hard to predict what accommodations experience might eventually show are best suited to tiny vehicles, they are unlikely to involve that same scale of trade-off.

Already tiny vehicles offer the benefit of fun. They promise serious benefits, too. Their advent may be the best thing to happen to the Vision Zero movement since the separated bike lane.

This article is published as part of the Vision Zero Cities Conference, November 7-8 in New York City. Register for the conference.

Peter Jacobsen is a professional engineer with a strong interest in the health impacts of transportation policy. His published work ranges from injury prevention to activity promotion. Joel Jacobsen is the author of two books and writes a regular business law column for the Albuquerque Journal.


"The Micro-Mobility Revolution: The Introduction and Adoption of Electric Scooters in the United States" Populus Research.
"Health Officials Prepare to Track Electric Scooter Injuries," Bradley Berman, The New York Times (Aug 2, 2018)
“Safety in Numbers: More Walkers and Bicyclists, Safer Walking and Bicycling.” Jacobsen, Peter. Injury Prevention
“Infrastructure, Programs, and Policies to Increase Bicycling: An International Review.” Pucher, John, Jennifer Dill, and Susan Handy. Preventive Medicine
“Global Bike Share: What the Data Tells Us about Road Safety.” Fishman, Elliot, and Paul Schepers. Journal of Safety Research
“Safety in Numbers for Walkers and Bicyclists: Exploring the Mechanisms.” Jacobsen, Peter Lyndon, David R. Ragland, and Charles Komanoff. Injury Prevention
“Safety in Numbers: Target Prevalence Affects the Detection of Vehicles during Simulated Driving.” Beanland, Vanessa, Michael G. Lenné, and Geoffrey Underwood. 2014. Attention, Perception & Psychophysics
“Evidence on Why Bike-Friendly Cities Are Safer for All Road Users.” Marshall, Wesley E., and Norman W. Garrick. Environmental Practice
"The non-linearity of risk and the promotion of environmentally sustainable transport." Elvik, Rune. Accident Analysis and Prevention.

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