SACRAMENTO — Scooter riders can let the wind flow through their hair — at their own risk, of course — after Gov. Jerry Brown signed a bill Wednesday allowing adults to decide whether to wear a helmet.

Brown signed AB2989 by Assemblyman Heath Flora, R-Ripon (San Joaquin County), which brings motorized scooters under the same legal requirement as motorized bicycles — only riders age 17 and under are required to wear a helmet. The bill was backed by one of the leading scooter rental companies, Bird.

Besides lifting the helmet requirement for adults, the bill authorizes cities to allow scooters on streets that have a speed limit of 35 mph. At present, scooters can be operated only on streets where the speed limit is 25 mph or less.

The scooters themselves are still limited to a top speed of 15 mph, and are still banned from sidewalks.

The bill prompted concern from some safety advocates who said it could lead to more scooter-related injuries. Christopher Colwell, chief of emergency medicine at San Francisco General Hospital, told The Chronicle this month that the hospital has seen riders who have been hit by cars and arrived with potentially fatal cranial bleeding. On one recent night, he said, he treated two scooter riders who hadn’t been wearing helmets and suffered concussions.

Flora, the bill’s author, said, “For me, it all goes back to personal responsibility.” He said motorized scooters were a tourist attraction for many cities and that some people who might otherwise use them would not if they had to wear a helmet.

San Francisco briefly banned scooters after startups Bird, Lime and Spin dropped hundreds of the rentals on city streets in late March, prompting complaints about abandoned scooters and unsafe riders. The city forced the scooters off the streets until the Municipal Transportation Agency could set up a permitting process.

Last month the transit agency chose companies Scoot and Skip to receive permits to operate in the city under a one-year pilot program that starts in October.

Scoot and Skip will set up a total of 1,250 scooters that people can unlock with their phones in order to cruise around the city.

Melody Gutierrez is a San Francisco Chronicle staff writer. Email: mgutierrez@sfchronicle.com Twitter: @MelodyGutierrez