Updated 11:40 a.m., 7/15/11.

Dr. Kevin Mack was on his way to work at San Francisco General Hospital Thursday morning, when the shuttle bus he was riding in collided with a big rig at the corner of Oak and Octavia.

Dr. Mack was ejected from the jitney-sized University of California San Francisco shuttle and "ended up underneath the big rig, but was not run over or crushed," said SFPD Lt. Troy Dangerfield.

The 52-year-old psychiatrist was pronounced dead at the scene.

Dr. Mack wasn't wearing a seat belt, because none of UCSF's 15-passenger shuttles have them -- and aren't required by law to have them. 

"Seatbelts are not required in vehicles meeting the definition of a bus," explained California Highway Patrol spokeswoman Jaime Coffee, adding that the legal definition was last changed in 1994.

According to the state's vehicle code, a bus includes any "vehicle designed, used, or maintained for carrying more than 10 persons, including the driver, which is used to transport persons for compensation or profit, or is used by any nonprofit organization."

Lorrie Walker, technical advisor to the Washington-based non-profit SafeKids, said the accident underscores the risks inherent in riding in small vehicles without a safety belt.

"Buses are so much bigger and have more weight and are less likely to have their passengers sustain serious injuries or damage," she said. "But you don't have that with these little jitneys that are just slightly larger than a car."

UCSF shuttles travel approximately one million miles a year and carry 2.3 million passengers annually, the campus said in a statement.

The California Highway Patrol regularly inspects UCSF's fleet and in October 2010 gave the shuttle system a "satisfactory rating."

The UCSF shuttle Mack was riding in was heading northbound on Octavia, while the big rig, which was carrying four mid-sized sedans, was heading eastbound on Oak, police said. The exact cause of the crash is under investigation.

According to a statement from UCSF, three other injured passengers – two women and a man between the ages of 58 and 85 – were taken to San Francisco General Hospital and are listed in fair to good condition.

The driver suffered minor injuries and was taken to UCSF. He has been placed on paid administrative leave.

Dr. Mack was an associate professor of psychiatry at UCSF and worked in SF General Hospital's psychiatric emergency room. According to the school, he was an expert in bipolar and psychotic disorders.

Dr. Renee Binder, the chair of the UCSF's Psychiatry Department, said faculty and students alike were "devastated" by the loss.

Dr. Mack was particularly dedicated to the education of his students, she said. He recently returned from Ethiopia as part of his commitment to global mental health.

Dr. Mack was at least the second person to die in an accident involving a UCSF shuttle within the past year. Last November, 65-year-old Nu Ha Dam was hit by a UCSF shuttle while she was crossing the street at Geary and Leavenworth.

According to UCSF spokeswoman Elizabeth Fernandez, criminal charges are pending against the shuttle driver in that case, while the driver remains on paid leave.

Fernandez said those are the only known fatal accidents in the shuttle services 30 year history.

Dr. Mack leaves behind a husband and two young children.