WALNUT CREEK -- An exhibition of dangerous driving by two motorists, one of them on a motorcycle, ended with one of them dead and another hospitalized Sunday, police said.

The horrific wreck at 5:48 p.m. on Interstate 680 just north of Alamo killed the motorcycle driver, who was dismembered, California Highway Patrol Sgt. Franco Dalao said.

A driver of a black two-door coupe that was destroyed in an ensuing car fire that was put out by members of the Contra Costa Fire Protection District, was taken to John Muir Medical Center with serious injuries, Dalao said.

Parts of the motorcycle, including what appeared to be a sizable piece of its midsection, was strewed for at least a quarter-mile stretch in the northbound direction of I-680 between Livorna and Rudgear roads, where the wreck occurred.

Traffic was backed up for miles, as all four lanes of the freeway were shut down for approximately 90 minutes. All were reopened shortly before 8 p.m., the CHP said.

Dalao said the investigation was ongoing, but that "reckless driving is a possibility. We do have several witnesses that reported that the motorcycle and the black coupe were going at a very dangerous rate of speed."

A Walnut Creek woman said the dead motorcyclist appeared to be racing the black coupe and that the motorcyclist appeared to be doing tricks.

"He had his right hand on the bar and his left hand on the seat," Alexandra Crowe said of the motorcyclist. "And he kept looking behind him."

The northbound stretch of I-680 just north of Livorna Road before Rudgear Road turned into a virtual parking lot as police investigated, with people sitting atop their roof tops, taking up posts on the hills above the highway shoulder and mingling on the road.

"We'd like this to be a reminder for people to slow down out there and look out for one another," Dalao said. "We hope that the people who are having to sit through this are reminded that the roadway is a dangerous place and why it's vital to be cautious instead of aggressive."

Rick Hurd covers public safety. Contact him at 925-945-4780 and follow him at Twitter.com/3rdERH.