What began with three dogs jumping from a car quickly escalated yesterday into a six-vehicle collision on a ramp between the Capital Beltway and Interstate 95 in Virginia -- a wreck that led to the midday closure of the ramp and two Beltway through lanes, tying up traffic for miles.

About 11 a.m., as a car was exiting the outer loop of the Capital Beltway to get onto Interstate 95 south, three dogs leapt from an open window and began running up the roadway.

An asphalt truck jackknifed. And by the end of the ensuing mayhem, a tractor-trailer and four cars had been involved, as well, Virginia State Police said.

There were no human fatalities, although some people wound up with serious injuries, and one dog was killed, police said. The two other dogs were taken to animal shelters.

"It was a chain-reaction accident," said Sgt. Wallace Bouldin, spokesman for the state police. "We're still investigating."

The lane closures, which for the most part lasted until the mid-afternoon, posed significant delays for travelers. Special teams for dealing with hazardous materials had to be called in because of diesel and gasoline leaks.

"The good news is that it didn't happen at rush hour," said Virginia Department of Transportation spokeswoman Joan Morris.

The accident came on a day that should have been pure celebration for drivers around the notorious Springfield interchange, known as the Mixing Bowl.

About an hour beforehand, a ribbon-cutting was held on a construction project that Virginia transportation officials have billed as the "most significant improvement" yet in their long-term effort to rebuild the complex interchange.

Traffic began flowing over a new 100-foot-high, mile-long overpass from the inner loop of the Capital Beltway to Interstate 95 south, a connection considered one of the key choke points there.

"This is probably the most significant improvement that we've given motorists since the project started in 1999," Morris said. "We still have three years to go."

One of the six vehicles involved in the collision is removed from the scene on a truck. The accident closed a Beltway ramp and tied up traffic for miles.