Traffic accidents and crash-related injuries and deaths were reduced by half during the first three weeks of California’s shelter-in-place order, which began March 20. The reductions save the state an estimated $40 million per day—about $1 billion over the time period—according to an updated special report released from the Road Ecology Center at the University of California, Davis.

The report, “Impact of COVID-19 on California Traffic Crashes,” compares hot spots for injuries and fatal crashes on state highways and major roads patrolled by the California Highway Patrol between 27 February 27 and 11 April 2020—the days leading up to and following the order—and the same period in 2019.

In parallel with the more than 50% reduction in traffic collisions and related injuries and deaths came a 55% reduction in traffic on some highways. There was also a 40-50% decrease in trauma-injuries for drivers, pedestrians and cyclists reported among Sacramento-area hospitals.

The downward trend in collisions was disrupted in early April when record-breaking rains in Southern California brought a brief return to “normal” levels of crashes, the report noted.

Crashes dropped roughly in half from March 21 to early April 2020, when heavy rains in Southern California brought a short-lived return to "normal" conditions.

Altogether, the reduction amounts to about 15,000 fewer collisions per month and 6,000 fewer injuries or fatal accidents per month that can be directly attributed to the shelter-in-place order.

The report notes that the reduction in traffic accidents is unparalleled. “There is no equivalent in our recent transportation history to such large changes in vehicle movement on our state and local roads.”

But it also notes that there is a key outstanding question: “Why are so many people still driving on what may be non-essential trips?”

We are so used to driving, I think it is difficult for people to stop, even with state and local orders compelling them to do so.

—lead author Fraser Shilling

To create the report, the authors used observations and reported traffic incidents from CHP in the real-time “California Highway Incident Processing System,” or CHIPS. The Road Ecology Center often uses state transportation data to track collisions between wildlife and vehicles, which it reports annually. The shelter-in-place order offered a new opportunity to explore unintended impacts of the order on state highway traffic conditions.

The report was co-authored by David Waetjen in the UC Davis Department of Environmental Science and Policy.