Express toll lanes on Interstate 25 in Denver have drawn triple the projected number of rush-hour commuters, giving the state $60,000 a month more in toll revenue than forecast.
The popularity of the toll lanes raises the chances the lanes could be extended from the Denver area to Boulder, on U.S. 36.
Car pools and buses use the express lanes free, but solo drivers willing to pay fees also can use them.
Fees range from 50 cents to $3.25 cents depending on the time of day and are collected electronically.
Planners initially figured 500 solo drivers would pay tolls during the average weekday rush hour, bringing in about $80,000 in toll revenue.
The transportation department is eyeing at least six other highway segments in metro Denver for toll lanes.