As cars become more fuel efficient and revenue from gas taxes drops, states are turning to modern toll systems that don't back up traffic to replenish funding for highway repairs.

"I describe the gas tax as a system at risk," said Ed Regan, a transportation consultant and senior vice president with CDM Smith in New Haven.

"The backbone of delivering revenue to fix the transportation system is based on a fuel we are discouraging use of," Regan said.

"It will not generate enough money to expand and maintain our highway system in the future."

The General Assembly's transportation committee was told Friday that states are moving to electronic toll systems that scan license plates and transponders as vehicles pass by.

The system does not require cars to slow down or stop, and invoices are later mailed to drivers for payment.

"The bottom line is we have to find another way to fund infrastructure," said state Rep. Tony Guerra, D-Rocky Hill, the co-chairman of the transportation committee.

"We do it either by raising taxes or providing another tax to pay for our roadways," Guerra said.

The state has identified about $16 billion in unfunded road and bridge repairs and improvements needed over the next 20 years. Connecticut's primary funding source for repairs is the now $1.2 billion Special Transportation Fund.

The transportation fund is mostly fueled by one of the nation's highest gas taxes, but revenue is dropping. The state expected $496 million from gas taxes during the 2012 fiscal year but the total was $3 million less.

Gov. Dannel P. Malloy's proposed $44 billion state budget for the next two years transfers additional money to the transportation fund and reduces spending to cover projected deficits.

Regan and Lowell Clary, a consultant who previously worked for the Florida transportation department, said states are using tolls to supplement gas tax revenue. They describe systems that can cover entire highways or only specific lanes, such as those reserved for high occupancy use.

Both said states are striking agreements with neighbors so drivers who receive invoices from a state they don't live in won't be able to register their car until out-of-state tolls are paid, the said.

The experts said federal credits are available to help offset construction of toll systems. The federal government must still give permission, but it's becoming easier to gain approval, they said.

"You have provided some alternatives to consider," said state Sen. Toni Boucher, R-Westport/Wilton, who added she plans to ask constituents if past opposition to tolls would lessen if traffic didn't have to stop.