CHICAGO (CBS) — It has been four months since Pace buses began roaring up and down the shoulders of Interstate 55, and the program is proving so successful that Pace is adding buses and extending one of the lines.

As WBBM Newsradio’s Bob Roberts reports, the Stevenson opened its shoulders to Pace express buses in November of last year, as part of a two-year experimental program during the morning and evening rush when the expressway is congested.

READ MORE: At Least 3 Killed, 30 Wounded In Weekend Shootings Across Chicago, Including Mass Shooting In Chatham

Pace officials are pleased by ridership increase, although not necessarily surprised. They predicted that faster runs and better on-time performance would mean more riders.

Spokesman Patrick Wilmot said that on-time performance on the two lines has jumped from 68 percent to 92 percent. He said that ridership has responded, increasing 67 percent on the more heavily-traveled Route 855, which connects Plainfield with North Michigan Avenue and 120 percent on Route 755, which has connected Plainfield with the near West Side Illinois Medical District, and will be extended to serve Union Station effective Monday, April 9.

In addition, Wilmot said, Pace is adding a new round trip on Route 855, leaving Plainfield at 6:37 a.m. with stops in Romeoville and Bolingbrook. The new outbound trip departs Superior and Michigan at 4:40 p.m.

READ MORE: Man Shot, Wounded In West Englewood; Police Vehicle Also Struck By Gunfire

Debris and stalled cars on the inner shoulders of I-55, the ones used by Pace buses, could create problems quickly, but Wilmot said the Illinois Department of Transportation has done a good job keeping shoulders clear. He called the results so far “very encouraging.”

Pace drivers are instructed not to go faster than 35 miles an hour while on shoulders, and to use them only if traffic is moving more slowly. Shoulder use by the buses is allowed only during weekday rush hours.

Shoulder riding for other motorists remains illegal. Wilmot said that since the pilot project began Nov. 14, neither line has had an accident on I-55.

The two lines are the test bed for a program that could grow to include shoulder-riding lines on portions of the Tollway system, as the roads are resurfaced.

MORE NEWS: Coronavirus In Illinois: 298 New Cases, Including 11 Deaths; 52% Of Adults Fully Vaccinated

One-way fares on the two I-55 express bus routes remain at $4.