A rental scooter is shown in April in Washington. (Matt McClain/The Washington Post)
Contributing columnist

There are few things more properly in the public sphere than urban transit. Yes, private transit — personal cars requiring expensive parking, limousines with chauffeurs — has always been a feature of urban mobility for the few. But publicly directed and publicly funded means of getting around our cities (such as public buses and subways, and publicly regulated taxis) have made modern urban life possible for the many.

That may be changing. The latest example of the paradigm shift is appearing on the sidewalks of Washington and other cities. Walk down any street in downtown D.C. and you will see them: electric scooters and dockless bikes — parked everywhere and nowhere in particular. This is “public” transit, available for use by subscribers to various private services.

The new two-wheeled transportation options are no casual amusement. In the past few weeks alone, private venture capital firms have invested hundreds of millions of dollars in their deployment at a time when subway systems in New York and Washington are struggling with operational woes and funding deficiencies.

Billions more are likely to pour in soon. Who decided that our urban transportation grid needed scores of buzzing scooters and free-range bikes, instead of (for example) newer and cleaner buses or better- ­ functioning subways? Who weighed the respective claims of youth-friendly scooter-filled sidewalks against the desires of senior citizens or the disabled for more accommodating passage in our public spaces?

The answers to these questions point to how much we have privatized “public” transportation and the subtle but important impact this shift can have.

The most significant element of this shift has been the massive explosion of private ride-sharing services, such as Uber and Lyft. These huge fleets have become alternatives not just to taxis but also to public buses and subways. A $7 UberX ride from point to point in a city has replaced a $2 subway trip for those with the means. The bottom line: If you have money and agility, there are many new ways to get around town using private services as an alternative to a public bus or subway.

Moreover, the shift to private-“public” transit will only accelerate in the future. Tech giants Uber and Google are working on driverless vehicles as the next big thing in urban transportation. While a public high-speed rail initiative launched by the Obama administration was torpedoed by political opposition, Elon Musk’s privately owned hyperloop and underground boring projects appear to be the “trains” of tomorrow.

None of these innovations are all good or all bad. They provide improved transit for some users and may reduce carbon emissions, while negatively affecting other urban dwellers and potentially eroding support for mass transit.

The point, however, is that we have largely shifted the decision-making about how people should get around our cities from public authorities and public investments to private companies and private investors.

The change reflects deliberate choices in public policy. During the Obama years, Republicans fought fiercely against any increase in public investment in mass transit and advanced rail systems. In 2016, decrying the crumbling state of our public infrastructure, President Trump promised that he would pass the largest infrastructure bill in U.S. history during his first 100 days in office. More than 500 days into his presidency, the bill still hasn’t even been drafted.

In the meantime, Trump’s tax cuts put a further squeeze on state and local efforts to pay for improvements on their own by repealing the state and local tax deduction under federal law — hampering the ability of those governments to raise increased revenue from their citizens to pay for needed projects and services.

Where does that leave us? In a “this is not the Onion” moment last week, Domino’s announced it was launching a new program whereby it would fill potholes and make repairs in cities where the state of the roads was slowing pizza deliveries. While this act of generosity should be applauded, what does it say about the state of civic resolve to maintain our public infrastructure when its condition turns not on public action but on the priorities of a pizza maker?

Laboring under constrained budgets and without promised help from Washington, state and local governments can do little but watch their roads and sidewalks become laboratories for transportation innovation that will benefit some but not others. Upscale gentrified areas will soon be rich with autonomous cars, electric scooters and dockless bikes — while other populations will struggle with aging buses and creaking subways.

Private investment and private innovation are great things. But private players are driven by profit motives, not public benefit. Regulation of these new modes of transport is one important means of ensuring that they serve the public interest; increased public investment in infrastructure and mass transit is also vital. The roads and sidewalks belong to all of us, and a robust public transportation system — fully accessible and usable by all people in all parts of a city — benefits all of us.