Above left are 200 people in 177 cars. They fill the street to the horizon. The other images are on bikes (upper right), buses (lower left), and on a streetcar (lower right). There is no getting around the fact that low-occupancy vehicles are a tremendous waste of street space. It does not matter if the vehicle is conventional, electric, or autonomous—geometry prevails.

This is why, when the Brooklyn Bridge was converted from part rail to all car in 1948, it went from carrying 400,000 people/day to carrying only 170,000 people/day. It’s why a single New York L train carries as many people per hour as 2000 cars, and an articulated bus equals 100 cars. Autonomous cars are a great supplement to transit, but, in congested places, they are not a solution to transit. This is why Ford is also investing in autonomous buses.

6) Don't rob transit.

This may be the greatest risk. In congested cities, replacing trains and buses with autonomous cars will cripple mobility. Now, if you are a city like Davenport, Iowa with no congestion and a very small transit population, AVs may present a solution. For Des Moines, they do not.

Unfortunately, just the prospect of future AVs is already threatening transit investment in certain American cities. The Mountain View City Council just voted down dedicated BRT lanes on El Camino Real due, in part, to the expectation of AVs making buses obsolete. The Michigan Taxpayers Alliance and others are picking up on AVs and Uber as the last great weapon to finally kill transit. Even DC’s Metro is threatened.

Meanwhile, Uber has set its sights on transit and is offering UberPool monthly memberships at lower cost than a transit pass—some say to put the conventional transit out of business. Uber admits that this pricing is well below cost and unsustainable, and thus, by definition, predatory. City leaders have the responsibility to teach their citizens about geometry—to teach them what even Elon Musk doesn’t seem to know—that a shift from transit to AV cars would reduce mobility.

7) Own the streets and own the data.

It sounds implausible, but there is a very real worry that AV providers will ask to buy certain city streets, or certain segments of city streets, and cities will take the money. This has precedents—like Chicago leasing its on-street parking to Morgan Stanley for 75 years. We need to remember that your city’s streets are its principal public spaces and, especially downtown, they perform many more jobs than just moving vehicles. They are places to walk, bike, access buildings, dine, converse, grow trees, protest, and much more, and they belong to us all.

I particularly like this recent quote from Adam Gopnik: “Cities are their streets. Streets are not a city’s veins, but its neurology, its accumulated intelligence.” Never sell that.

Another ownership issue has to do with all the traffic data collected by Uber, which cities can benefit from in many ways—for example knowing which way to send a fire truck. Like Uber, AVs will represent a viable business model only by running on public streets. Sharing full data would seem a small price to pay for that privilege.

8) Don't buy any urban vision that forgets urbanism.

If you study modern urban history, you will see that every major transportation advance has brought with it whole new concepts of what the city is. We are a creative species and we can’t help but use any excuse to reinvent the human habitat. The problem is that all of these inventions, from Le Corbusier’s “Tower in the Park” to Frank Lloyd Wright’s Broadacre City, have ended up making our lives worse.