The shuttle was taking Mack and about 15 others to work at San Francisco General Hospital around 6:20 a.m. when it collided with a big-rig that was headed north on Octavia Boulevard at Oak Street in the city's Hayes Valley neighborhood, police Lt. Troy Dangerfield said.
Mack, 52, was ejected in the crash and landed underneath the big-rig, which was carrying several cars, Dangerfield said. He was pronounced dead at the scene. Three other passengers -- two women and a man -- were injured in the crash and were taken to San Francisco General Hospital for treatment.
Mack's peers remember him as an inspiration to students and faculty members who reminded them to consider the bigger picture.
"He had a strong commitment to global health and to medical education in resource poor settings," said Sue Carlisle, associate dean at the UCSF School of Medicine at San Francisco General Hospital.
Mack looked beyond his work at UCSF, where he was an advisory college mentor, to identify mental health needs in other communities.
"He worked with the World Health Organization developing problem-based learning programs for use in Africa and the South Pacific," UCSF School of Medicine Dean Sam Hawgood said.
Revered by colleagues as a gifted educator and an expert in bipolar and psychotic disorders, Mack also served as the director of Educational Technology and Faculty Development in the UCSF-Berkeley Joint Medical Program.
Mack studied medicine at the University of Hawaii, and completed his residency at Harvard, according to his faculty biography at the UCSF website, where he described himself as an educator first, and a doctor second.
"I'm basically an educator at heart, who chose medicine as a career after deciding that gay teachers weren't going to be 'safe' professionally for several decades," he wrote.
Mack is survived by his husband and their two children.
"Although I enjoy my clinical work, my greatest joy comes from helping people use their own sense of wonder and inquiry to find and fuel their passion -- whether that be in science, arts or something else entirely," Mack wrote.
The three other victims range in age from 58 to 85 years old, and all are in fair or good condition, hospital spokesman Tristan Cook said.
Assistant Fire Chief Art Kenney said the shuttle bus driver was also later taken to the hospital with minor injuries, but the driver of the big-rig was not injured.
It appears the shuttle was headed east on Oak Street just before the crash, Kenney said.
The crash is still under investigation by police.
The big-rig suffered damage to its left front end. The front right side of the shuttle bus near the door was crushed inward and a side window was broken.
San Francisco Supervisor Ross Mirkarimi, whose district includes Hayes Valley, was at the scene this morning.
Like police, he was still trying to learn the circumstances of the crash but said he is aware of the dangers of Octavia Boulevard.
He said Octavia is "a fabulous boulevard, but it has its challenges, especially for drivers who aren't familiar with San Francisco and rely on GPS."
Mirkarimi also brought up the issue of seat belts on shuttle buses.
In today's crash, he said, "Quite clearly a seat belt could have potentially helped."
Kenney said the UCSF shuttle did not have seat belts.
Octavia Boulevard was shut down between Market and Fell streets after the crash, and remained closed until shortly after 11 a.m.