Some drivers panic and flee. Some leave because they are drunk, and others run because they aren't licensed to drive.
But all hit-and-run drivers share a similar motivation, say traffic investigators: self-preservation.
"They're worrying more about what's going to happen to them rather than worrying about the person they hit," said Petaluma Police officer Ron Klein.
A rash of hit-and-run collisions that have caused deaths or major injuries in Sonoma County has highlighted the callous nature of the crime.
Three Rohnert Park drivers in less than a month have been accused of leaving the scenes of serious accidents — two since Friday.
The collisions caused two deaths — a teen-ager and a retired college professor — and left a professional bike rider with critical injuries, according to CHP reports.
The men currently face a variety of charges including felony hit-and-run.
Not all such cases lead to arrests.
Frank Moreno, 49, of Santa Rosa was hit as he rode a bike across Fulton Road last November. Nearly a year ago San Francisco attorney Don Casper, 63, was hit from behind and killed as he jogged along River Road. No arrests have been made in either case.
And the identity of the driver of the maroon SUV that hit and critically injured a San Francisco man riding in the 2010 Gran Fondo bike ride remains unknown.
The crimes grate on the public's sense of humanity, say traffic officers who investigate such cases.
"A lot of times people just walk away. How do you do that?" asked Sgt. Lance Badger, a Santa Rosa traffic officer. "How could somebody do this .<TH>.<TH>. see this and walk away?"
Sonoma County's three main traffic enforcement jurisdictions — CHP, Santa Rosa police and Petaluma police — reported 581 hit and run offenses in 2011, 119 of which involved death or injury.
In Santa Rosa in 2011, 417 drivers involved in collisions left the crash scenes, according to department statistics. Of those cases, three involved deaths and 91 caused lesser injuries.
An injury elevates a hit-and-run to a potential felony case. For Santa Rosa in 2011, the District Attorney's Office filed felony charges against 16 drivers and misdemeanor charges against 266, according to police reports. In 135 cases, the driver was never apprehended.
"The hit and run drivers we don't catch, we have no idea why they run," said Badger. But time and again officers see the same themes for why people run.
"Nine times out of 10 somebody leaves the scene because they are unlicensed or under the influence or lacking insurance," said CHP Officer Jon Sloat. "Nine times out of 10 there is another crime being committed."
Drivers who have been drinking or using drugs and leave the scene sometimes want to sober up before talking to authorities, said officers.
That can be a losing proposition. A felony drunken driving conviction can result in a three-year prison sentence. Felony hit-and-run carries a potential four-year prison term.
A misdemeanor hit-and-run conviction, which means no injury was involved, carries for a one-year maximum sentence.
If a DUI is suspected in a hit-and-run case, officers try to retrace the steps of the driver to determine whether they were seen drinking, how much they drank and if witnesses can help prove the driver was impaired.