San Francisco is trying to slow the expansion of robotaxis after repeated incidents in which cars without drivers stopped and idled in the middle of the street for no obvious reason, delaying bus riders and disrupting the work of firefighters.

The city’s transportation officials sent letters this week to California regulators asking them to halt or scale back the expansion plans of two companies, Cruise and Waymo, which are competing head-to-head to be the first to offer 24-hour robotaxi service in the country’s best-known tech hub.

The outcome will determine how quickly San Francisco and possibly other cities forge ahead with driverless technology that could remake the world’s cities and potentially save some of the 40,000 people killed each year in American traffic crashes.