A NEW kind of vehicle is taking to the roads, and people are not sure what to make of it. Is it safe? How will it get along with other road users? Will it really shake up the way we travel? These questions are being asked today about autonomous vehicles (AVs). Exactly the same questions were posed when the first motor cars rumbled onto the roads. By granting drivers unprecedented freedom, automobiles changed the world. They also led to unforeseen harm, from strip malls and urban sprawl to road rage and climate change. Now AVs are poised to rewrite the rules of transport—and there is a danger that the same mistake will be made all over again.

AVs are on the threshold of being able to drive, without human supervision, within limited and carefully mapped areas (see special report). Waymo, the self-driving-car unit of Google’s parent company, hopes to launch an autonomous “robotaxi” service in the suburbs of Phoenix, Arizona later this year. General Motors, America’s biggest carmaker, plans its own robotaxi service for 2019. On February 26th California said it would abolish the rule that experimental AVs must always have a safety driver on board ready to assume control.

Clean, dream machines

Assuming the technology can be made to work as AV firms expect, it is not hard to imagine the beginnings of the driverless era. Cost means that self-driving vehicles will at first serve as robotaxis, summoned using a ride-hailing app. That way they get used more, offsetting their costs, and provide transport that is cheaper per mile than owning a car, undermining the case for car ownership, at least for townies. UBS, a bank, reckons that urban car ownership will fall by 70% by 2050. Today’s cars sit unused 95% of the time, so a widespread switch to robotaxis would let urban land wasted on parking be reallocated.

AVs would dramatically reduce the number of road deaths and, being electric, cut harmful emissions in places with clean grids. Clever routing, closer spacing between vehicles and dynamic congestion-charging could cut traffic. Like cars before them, AVs will reshape cities (a long commute is easier if you work or sleep en route) and redefine retailing (shops can come to you). Carmakers will face enormous change (see article); instead of selling to individuals, they will supply fleet operators, or reinvent themselves as “mobility service” providers.

Economists and urban planners should rejoice because AVs mean that, for the first time, the unwelcome externalities associated with cars can be fully priced in. In particular, dynamic road-tolling and congestion charging, adjusting the cost per kilometre according to the time of day, level of traffic, length of trip and so on, will allow fine-tuning of entire urban-transport systems. By setting taxes and tolls accordingly, planners can subsidise rides in poor districts, for example, or encourage people to use public transport for longer trips. They can also ensure that the roads do not end up full of empty vehicles looking for riders. Such granular road-pricing is the logical conclusion of existing schemes. Some cities already have congestion-charging regimes, subsidise ride-hailing in poor areas ill-served by public transport, or impose per-ride taxes on Uber, Lyft and their kind.

Yet the same tolling schemes that will let city planners minimise congestion or subsidise robotaxi services in underserved “transport deserts” have a darker side—and one to which too little attention has been paid. AVs will offer an extraordinarily subtle policy tool which can, in theory, be used to transform cities; but in the hands of authoritarian governments could also become a powerful means of social control.

Panopticons on wheels

For a start, AVs will record everything that happens in and around them. When a crime is committed, the police will ask nearby cars if they saw anything. Fleet operators will know a great deal about their riders. In one infamous analysis of passenger data, Uber identified one-night stands. If, as seems likely, human-driven cars are gradually banned on safety grounds, passengers could lose the freedom to go anywhere they choose. The risk that not all robotaxis will serve all destinations could open the door to segregation and discrimination. In authoritarian countries, robotaxis could restrict people’s movements. If all this sounds implausible, recall that Robert Moses notoriously designed the Southern State Parkway, linking New York City to Long Island’s beaches, with low bridges to favour access by rich whites in cars, while discriminating against poor blacks in buses. And China’s “social credit” system, which awards points based on people’s behaviour, already restricts train travel for those who step out of line.

So as robotaxi services roll out this year, and expand to cover wider areas in more cities in the years to come, there is more to think about than technology and transport policy. Experiments with different pricing schemes, decisions about whether to ban private vehicles from city centres, and license auctions for competing private robotaxi operators sound harmless enough. But collectively they represent a seismic shift for society. Autonomous vehicles offer passengers freedom from accidents, pollution, congestion and the bother of trying to find a parking space. But they will require other freedoms to be given up in return—especially the ability to drive your own vehicle anywhere. Choices about who can go where, when and how are inescapably political in nature.

A century ago cars were seized upon as a solution to the drawbacks of horses, which were clogging city streets with manure. The broader social consequences of cars, both good and bad, were entirely unforeseen. Today the danger is that AVs will be treated merely as a technological solution to the problems associated with cars and that, once again, the wider impacts will be overlooked. AVs have the potential to transform physical transport as radically as packet-switching transformed the delivery of data. But as with the internet, realising their benefits is a matter of politics as well as technology. AVs offer a chance to forge a new and better trade-off between personal mobility and social impact—but only if the lesson of the horseless carriage is applied to the era of the driverless car.