SAN DIEGO -- Solo drivers who use the FasTrak automated tolling system to access the carpool lanes on Interstate 15 will see a change in the way the charges are calculated starting this Saturday when SANDAG initiates the launch of a new distanced-based dynamic pricing system, authorities said Thursday.

According to a SANDAG news release, FasTrak customers accessing the Express Lanes will now pay a toll based on the distance they travel in the lanes and their entry location.

Every few minutes, the new state-of-the-art system will recalculate the per-mile toll rate based on the level of traffic in the I-15 corridor, making sure traffic flows freely in the Express Lanes, SANDAG officials said.

The toll rate FasTrak customers see on the signs located just before each of the entrances to the Express Lanes will provide the minimum toll that they can expect to pay if they enter the facility regardless of their eventual exit location.

The signs also will advise them of one or more possible fares for longer trips to upcoming freeway interchanges, such as State Route 56 or 163.

If a driver is traveling to a destination somewhere between the first possible exit point and the next major freeway interchange, that person can expect to pay a toll that falls between the minimum and the amount to the interchange, SANDAG officials explained.

SANDAG described the new FasTrak system as "the most advanced in the world."

The toll displayed on the sign when a car enters will apply for the entire trip – even if the rate goes up or down while that customer is in the lanes.

A sophisticated identification system using the FasTrak transponder that customers have in their cars will track where and when they enter and exit the lanes, automatically calculate their toll, and flag violators for enforcement by the California Highway Patrol.

The FasTrak system is one element of the Express Lanes on I-15, which are being expanded to four reversible lanes stretching 20 miles from SR 163 to SR 78.

The first section of new express lanes opened north of SR 56 in September 2008.

Another 3.5 miles will debut soon, SANDAG said.

In addition, construction continues on four new miles in the north and the expansion of the existing eight miles in the south.