Utah legislators raised questions Wednesday about a proposal to double the tolls on Interstate 15 express lanes — saying they may need to be even higher to cut congestion. Alternatively, they raised the prospect of requiring carpools to have three people instead of two for free use of express lanes.

State highway officials last month proposed doubling the flexible tolls — upping the $1 maximum per section charge to $2, depending on traffic. The Utah Transportation Commission is considering the plan, and may vote as early as next week.

But the Legislature has final say and the issue sparked questions Wednesday in the Revenue and Taxation Interim Committee.

Shane Marshall, deputy director of the Utah Department of Transportation, said the primary purpose of the express lanes is to encourage people to carpool. It does that by trying to maintain speeds of at least 55 mph as an incentive for people to double up.

“Our goal is to put more people in fewer cars so the general-purpose lanes function better,” he explained.

But UDOT has allowed 20,000 people to buy access for single-occupant vehicles to the express lanes. The program, using transponders to exact electronic charges, was originally designed to utilize excess capacity. The trouble is, too many people are now enrolled — and the $1 maximum toll isn’t enough to keep the express lanes congestion-free.

Every weekday afternoon in Salt Lake County between 1300 South and the Utah County line, express-lane speeds start to drop below the 55 mph goal by about 4 p.m, says UDOT. They bottom out at 31 mph on average by 5:15 p.m. and don’t rebound past 55 mph again until nearly 6:30 p.m.

Sen. Howard Stephenson, R-Draper, Senate chairman of the committee, asked the same question that some transportation commission members posed last month — why not raise the toll even more than the proposed $2 maximum per section?

He encouraged considering “charging the maximum the market will bear” not only to reduce congestion, but perhaps to raise some money for highways. Several transportation commission members earlier suggested a maximum toll of $3 or even $5, saying even $2 was too low to really reduce congestion.

UDOT’s Marshall, however, said the agency isn’t sure what that “maximum the market would bear” is — so it prefers to raise the toll initially to $2, and then study how well that helps to control congestion. If results fall short, then UDOT would consider bigger hikes.

Marshall said UDOT isn’t seeking to make money through tolls, only to cover costs of that program and help keep down congestion enough so that carpoolers may travel at least 55 mph.

Stephenson also asked if UDOT has considered requiring at least three occupants — instead of two — to carpool in the express lane for free. He said that might relieve congestion and encourage even larger carpools.

UDOT hasn’t looked into that option, but may over time, said Marshall.

Stephenson was also distressed to hear that up to 20 percent of cars in express lanes now are there illegally. He said reducing that through enforcement may solve the congestion problem.

Marshall explained that some of the toll money collected is used by UDOT to pay for extra Highway Patrol shifts to catch violators, and also for educational programs about who should be there.

Rep. Karianne Lisonbee, R-Clearfield, raised one more option for consideration — eliminating the toll-paying, single-passenger cars from express lanes to allow more use by clean-fuel vehicles.

UDOT has placed a cap of 6,650 “C decals” for purchase to allow clean-fuel, single-passenger cars into the express lanes for free. UDOT has already hit that limit — and has a waiting list of 1,065 people.

Lisonbee said cutting toll users to allow more use by clean-fuel cars may help relieve congestion for carpoolers while also helping to reduce pollution.