A cable par passes a line of taxicabs in front of the St. Francis Hotel in San Francisco. Photo: Justin Sullivan via Getty Images

Conventional bus and train transit excels at moving large volumes of passengers along busy corridors, but struggles to provide cost-effective service in outlying areas, where circuitous bus routes operate with few hourly boardings. At the same time, on-demand transportation — Lyft, Uber and any of their future self-driving iterations — converges onto those busy corridors and exacerbates congestion. This mix creates unserved “transit deserts" with high need for public transportation but little availability.

The big picture: A more efficient system would better tailor transit technologies to their environments — with buses and trains on heavily traveled routes and on-demand transportation in low-density areas — and integrate sparse areas with mainline service to overcome transit’s perennial "last-mile problem" at the beginning and end of trips.

Ideally, travelers should be offered transportation services that match their trips. A trip along a busy stretch and ending downtown would be taken in trains or buses operating in exclusive lanes. Travel between two points in the outer suburbs might rely entirely on on-demand transportation shared between unrelated parties, akin to Lyft Lines or Uber Pool. Trips that span the territories would connect service types, increasing the usefulness of mainline transit to people whose origins and destinations are not within walking distance of the bus or train.

Reality check: A system like this will not emerge by itself: Transit operators will fall back on familiar service models, and on-demand transportation will tend to duplicate, rather than complement, mainline transit.

What to watch: Avoiding those pitfalls requires both carrots and sticks. Disincentives might include congestion pricing and access restrictions, such as high-occupancy vehicle lanes. Incentives for serving underserved areas and linking them with mainline transit require public investment. For example, transit agencies might operate on-demand service themselves, contract for such services directly or make subsidies available when qualified trips are booked.

Jonathan Levine is a professor of urban and regional planning at the University of Michigan.

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