Computers will now set the price to drive on the express lanes on Interstate 15, and those prices could go up or down every three minutes depending on how congestion there is on the freeway.

But motorists are being assured the prices will be posted and will not change on them after they enter the toll road.

The center median of Interstate 15 between Escondido and the northern fringe of San Diego has been converted from a reversible carpool route into a bi-directional tollway-inside-a-freeway. And the tolls for the 12.5 mile segment now open between Rancho Bernando and Kearney Mesa will run anywhere from 50 cents to $8, depending on traffic volume, Colleen Windsor, a spokesperson for the San Diego Association of Governments, told the North County Times.

Toll rates will be adjusted as often as every three minutes, in response to increasing or decreasing traffic volume.

To use the lanes, commuters drive onto them at any several entrance points without going through the traditional tollgate, like those on some Orange County tollways. Instead, users purchase wallet-sized FasTrak transponders from the association that automatically deduct charges from prepaid accounts as they enter the express lanes.

Windsor told the Times drivers need not worry about their fees changing on them after they enter the express lanes, even though posted rates may change in the middle of their trips.

"Whatever you start with is your rate for the entire trip," she said.

And, if there are two or more people in a vehicle, the Times reported, the motorist does not pay anything -- and he or she does not have to have a transponder -- to use the lanes.

This weekend's change will bring the policy for 4.5 miles of new lanes north of Highway 56 in line, the Times reported, with the 8 miles of express lanes south of there, which have been operating for years with tolls that change with traffic volume.

The final 3.5 miles of express lanes under construction between Rancho Bernardo Road and Via Rancho Parkway on Escondido's south side are expected to open March 16, Antoinette Meire, a state Transportation Department spokesperson, told the Times.