As the number of bicycles continues to grow on Bay Area streets, so do the conflicts between cars and bikes, and they're becoming a topic of anger in both camps.

The problem is particularly pronounced at night, when vehicles are accustomed to having the roads to themselves, but they are suddenly confronted with bicyclists who are almost invisible, until it's too late.

On one Friday night on trendy Valencia Street in San Francisco, young people were on the move just as bars were getting ready to pour.

As twilight descended, the most vulnerable of the crowd dodge dangers on every block. There were bicyclists without helmets, some without lights -- and then, there were those without either.

For bicyclists 16 and older, San Francisco County has the highest number of victims killed and injured in all of California.

Figures from a database collected by the California Office of Traffic Safety shows that in 2010, the most recent figures available, almost 600 people were killed or injured on bikes, making it the most dangerous California county for biking when ranked by daily vehicle miles traveled.

Dan Cook has been biking Bay Area streets for a quarter of a century.

He was riding his bike in Oakland when KTVU stopped him to get his perspective on bike riders without helmets or lights.

"I think they're insane," Cook said. "I think they really are insane."

KTVU stopped Helena Gannon, one of the many younger bike-riders on Oakland streets. She told KTVU why she doesn't wear a helmet or have lights, even at night.

"It often feels like a benefit to be hidden," Gannon said. "I've heard of people being mugged on their bikes. Like, people just run off the sidewalk, knock them over and take whatever they have."

"I was actually chased. A guy chased me, with a stick, off my bike. He came from a bike stop and swung a metal pole towards me, and I was able to swerve, and luckily there was no traffic. But it's pretty common," she said.

In the South Bay, Santa Clara County had the fourth-worst number of bicyclists killed or injured among California counties when rated by daily vehicle miles traveled -- almost 800 victims in 2010 alone.

KTVU spoke with riders near San Jose State University to ask why they don't use lights or helmets.

"No one wears helmets in San Jose," said one rider.

State law requires bicyclists under the age of 18 to wear a helmet. The law also requires all bicyclists to have a white light in front and on the sides visible for 300 feet, a red rear reflector visible for 500 feet, and yellow or white reflectors on the pedals, shoes or ankles visible for 200 feet.