After discussing it for nearly a year, the Utah Department of Transportation is doubling the maximum toll for express lanes on Interstate 15 beginning Saturday.

The maximum for drivers using electronic transponders has been $1. It now will rise to $2 for segments of about 10 miles in length. Tolls vary according to how heavy traffic is. The minimum toll during light-traffic times is 25 cents per segment.

The primary purpose of express lanes is to encourage people to carpool. Use of those lanes is free to vehicles with at least two passengers, and UDOT strives to maintain speeds of at least 55 mph there as an incentive for people to double up.

It has, however, allowed 20,000 people to buy access for single-occupant vehicles. The program, using transponders to collect electronic charges, originally was 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.

Legislators this year questioned whether even doubling maximum tolls will be enough to reduce congestion in the express lanes.

Sen. Howard Stephenson, R-Draper, chairman of the Revenue and Taxation Interim Committee, last year suggested “charging the maximum the market will bear” not only to reduce congestion but also to raise some money for highways.

Members of the Utah Transportation Commission also said a $2 maximum is not enough to reduce congestion and suggested making it $4 or more.

Amid such concerns, the Legislature this year approved raising the tolls to as much as $4.

But UDOT said in a news release this week that it has decided to raise the maximum to $2 for now and to study how well that works before possibly raising them higher.

The agency also said that all toll revenues will continue to be used only to maintain the express lanes, and to improve technology and law enforcement associated with their operation.

The change comes after UDOT studies last year showed that express lanes are not exactly expresslike lately. Average speeds have dropped to 31 mph during peak drive times in Salt Lake County. Federal regulations require keeping average speeds in them at no less than 45 mph.

For example, every afternoon in Salt Lake County between 1300 South and the Utah County line, speeds start to drop below the 55 mph goal by about 4 p.m. 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.

UDOT has said tolls are now about 10 cents per mile on average and would double to 20 cents.

It said that is far less than the 87 cents toll per mile charged on average in other Western states. Some examples range from a low of 43 cents a mile on Interstate 880 in California to $1.10 a mile on California’s Interstates 10 and 110.

UDOT has said that boosting tolls is just one of many steps that it has taken or plans to take to reduce crowding in express lanes.

For example, it has placed a cap of 6,650 “C decals” for purchase to allow clean-fuel, single-passenger cars into the express lanes. UDOT has already hit that limit and has a waiting list of more than 1,000 people hoping to buy a C decal someday if current holders sell their cars or decide not to renew them.

UDOT also funds occasional blitzes by the Utah Highway Patrol to ticket drivers who are in the express lanes illegally. Past studies by UDOT consistently found that about one of every five cars there were violators.