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In my mind, the model city of the 21st century is Venice. It is a city on a human scale, where you walk without fear of being flattened by a truck, talk on the streets without raising your voice and breathe without ingesting fumes from gasoline or diesel, because there are no cars.

Venice is still such an anomaly that it is one of the most popular tourist destinations on the planet—far more so than copycat versions in Las Vegas and China. But don't give up hope, because big tech companies are promising an urban revolution that's the next best thing to a modern-day Venice: cities where privately owned cars with internal-combustion engines give way to fleets of self-driving electric cars that will greatly reduce urban congestion, pollution and noise, and free up parking lots for housing or green space.

This vision, brought to you by the developers of the robo-vehicles—Alphabet (the owner of Google), Tesla and Uber among them—is a fantasy, of course, but one that is sending mayors and urban planners into a state of rapture. There isn't enough discussion about the possibility that all those driverless cars could make cities less livable, not more. The tech companies want to sell product. The potential for urban dystopia doesn't exactly figure in the corporate literature.

North America and European cities were shaped by the automobile after the Second World War. Entire neighbourhoods were mowed down to make way for expressways and elevated connector roads, all the better to funnel vast streams of cars downtown, then back out to the suburbs. City centres began to die, while suburbs flourished.

Related: Self-driving cars will dramatically change our world, so when does the revolution begin? A seven-part Globe series

Since then, the tomorrow-land concept of cities shaped by individual transportation in cars has retreated, but only somewhat. Urban planners came to realize that new roads could only ease traffic for a while, before attracting even more traffic, and that the cost of building and maintaining roads would never be recouped from the car-driving public. The emphasis was put back on collective transportation systems—subways, streetcars, light rail and commuter trains.

The arrival of the self-driving car threatens to increase car dependency once again. In the new era of densely packed megacities, the last thing we need is more cars and less public transportation.

But the principal allure of self-driving vehicles—convenience—is awfully strong. Booked through an app, a car rolls to the front of your house or apartment within minutes. You stuff the kids in and they are taken off to school. Another car takes you to work. Repeat the process at the end of the day. While it's not cheap, it's cheaper than a cab, because there is no driver to pay.

In theory, cities would become decongested, because fewer cars would be used by more people. The average private car sits idle 95% of the time. A driverless car would be used most of the day as it moved from one booking to another.

The theory could be dead wrong. The first dubious assumption is that driverless cars will be shared. Car-sharing programs have been around for more than two decades in many cities, yet their market share is minuscule. Many driverless cars might be privately owned, meaning they, too, might sit idle most of the time.

It is also quite possible that families will use their cars more because they are so convenient. In a 2016 report on urban mobility, the consulting firm McKinsey & Co. and Bloomberg raised the prospect of an urban nightmare: "With lower marginal costs to travel an extra mile in an EV [electric vehicle], and without requiring a driver's attention thanks to autonomy, the demand for mobility could increase and thus add to congestion. Passenger miles travelled could grow 25% by 2030, with the majority attributable to additional autonomous travel in private vehicles."

As a result, public transportation could very likely suffer. Therein lies the nightmare scenario. Even in the centre of big cities like New York, Toronto, London and Paris, you often have to walk 200 or 300 metres to the nearest metro or bus stop. It's easier to have a car come to your doorstep. But that would clog secondary streets. It would also make you fatter—various studies have shown that public transportation promotes better health.

Imagine the congestion if public transportation use were to fall by even a quarter in big cities. More roads and parking garages would have to be built. Suburbs would expand, because if you could watch a movie in the car on the way home, the hour-long commute would be less unpleasant.

Urban life would suffer, too. Since the 1970s, mayors and urban planners have been trying to give city centres back to the people. Investments were made in transit and bike lanes, and entire streets were closed to traffic. The advent of driverless cars threatens to upend this progress. Their success could send cities back to the multilane, car-park hell of the 1950s and 1960s.

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Matt Bubbers speaks with Toronto's chief planner and Toronto's manager of transportation services to see how Canada's largest city will look different in 20-40 years

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