Shared Mobility in Cities: What Works According to Shaheen and Cervero

Shared mobility is the set of services that let people use a vehicle without owning it: cars by the hour, ride-hailing, bicycles and scooters rented by the minute, on-demand transit and platforms that combine all of them in a single app. It was born with the car-sharing cooperatives of Switzerland and Germany in the 1980s, professionalised with Zipcar in 2000 and exploded with smartphones from 2010. Two researchers have measured its effects most rigorously: Susan Shaheen, of the University of California, Berkeley, the world's leading reference on car sharing, and Robert Cervero, a specialist in the relationship between transport and urban form.

Susan Shaheen and Robert Cervero: what car sharing measures

Shaheen has documented since the 1990s the effect of classic car sharing, the car booked by the hour and picked up from a fixed bay. Her North American surveys with Elliot Martin estimated that each shared vehicle removes between nine and thirteen private cars from circulation, because many users sell theirs or give up buying one, and that households who join drive fewer kilometres and use public transport, cycling and walking more. Cervero evaluated San Francisco's City CarShare programme between 2001 and 2005 and found the same pattern: less ownership, fewer kilometres and less parking demand in the neighbourhoods served.

Uber and the platforms: more kilometres, not fewer cars

Platform ride-hailing, Uber, Lyft, Cabify, Didi, changed the balance. Shaheen and other researchers found that, unlike car sharing, these services add traffic: between 40 and 60 percent of their trips replace journeys that would have been made by public transport, on foot or by bicycle, or would not have been made at all, and the cars drive empty between rides. Bruce Schaller's studies in New York and those of San Francisco's transport authority attributed a substantial part of the rise in congestion since 2010 to the platforms. The technology that promised fewer cars produced, in that segment, more kilometres.

Micromobility and bike share: the favourable balance

Micromobility, free-floating bicycles and scooters since 2017, and public bike-share systems since Paris's Vélib in 2007, have a more favourable balance: they replace mostly short car and taxi trips, feed public transport by solving the first and last kilometre, and take up a fraction of the space. Their problems are the regulation of parking on pavements, safety and the financial sustainability of private operators, which have frequently gone bankrupt or withdrawn from cities. Public or concessioned systems with service obligations have proved more stable than free-market ones.

Urban form and regulation: when sharing shrinks the fleet

Cervero places all this within urban form. Shared mobility works where there is density, mixed use and public transport to complement it; in the dispersed city it becomes a more expensive taxi and more cars. And its effect depends on policy: if platforms pay for kerb use and emissions, if parking is regulated and made more expensive, if the space freed by cars is returned to pavements, bike lanes and public transport, shared mobility shrinks the urban fleet; if not, it grows it. Cities that have integrated the services into a public mobility-as-a-service platform, with Helsinki as pioneer since 2016, show the direction.

The shared mobility revolution is real but not automatic. Shaheen and Cervero agree that sharing vehicles can reduce car ownership, parking and emissions, and that it only does so when the city governs it: regulating platforms, giving space and priority to efficient modes and treating the kerb as the scarce public good it is. The question is not how many apps a city offers but how many private cars it stops needing.