CDOT is raising the toll on the I-25 Express lanes by a quarter, to $3.50, during the peak rush hour but not because demand is up.

The increase comes from CDOT's agreement with RTD never to set the toll lower than the express bus fare.

RTD fares are going up across the board next week, and express service, used mostly by commuters, is increasing from $3.25 to $3.50.

The I-25 Express Lanes operate on the Downtown Express car-pool and bus lanes that opened in 1994 in the center of Interstate 25 between downtown and U.S. 36. In 2007, CDOT began allowing solo drivers willing to pay a toll to escape I-25 congestion to use the Express Lanes. The prices vary according to time of day.

The $3.50 toll will be charged from 7:15 to 8:15 a.m. and from 4:30 to 6 p.m. That is when traffic is busiest. Toll charges ramp up and down on either side of the peak, from a base charge of 50 cents off-peak, to $1.25, $1.50, $1.75, $2 and $2.75 before hitting the top charge.

The agreement with RTD for setting the peak toll was reached because the lanes were funded in part by the Federal Transit Administration to facilitate bus service and car-pooling.

CDOT's plan always called for toll increases when the number of solo drivers rose to a level impeding traffic flow for buses and car pools. The idea is that raising the toll would control the level of traffic. But CDOT's toll traffic hasn't yet reached the level required to raise the toll, absent RTD's fare increase.

In fact, toll traffic was down more than 15 percent in November compared to the same month a year earlier. Car-pool traffic was off more than 16 percent.