Weekend motorists crawling through Interstate 70 in Clear Creek County already face plenty of obstacles, including crowded lanes, unexplained slowdowns and accidents.

But the newest challenge to drivers comes from above.

Ziplines shooting thrill-seekers back and forth between six-story towers in the Dumont to Idaho Springs areas are causing gawking among motorists on eastbound I-70, particularly in the Downieville-to-Dumont corridor.

It’s clogging traffic on the weekends and making a bad situation worse, according to the Colorado Department of Transportation.

“We know slowing along that part of I-70 is greater than it was before the ziplines went in,” said Stacey Stegman, CDOT spokeswoman. “And we are getting plenty of feedback from the public that traffic is slower there than this same time last year.”

CDOT compared travel times from July and August 2011 with the same times this year and found commuting lags in some instances.

On Sunday, July 31, 2011, at 1 p.m., it took 31 minutes to get from Georgetown to Idaho Springs, and the average speed was 24 mph. On Sunday, July 29, 2012, at 1 p.m., that same trip — when the zipline was operating — took 44 minutes with an average speed of 16 mph, according to CDOT.

On Sunday, July 24, 2011, at 1 p.m., the trip from Georgetown to Idaho Springs took 32 minutes and the average speed was 22 mph. But on Sunday, July 22, 2012, at 1 p.m., the same trip took 43 minutes and the average speed was 17 mph, according to CDOT figures.

There is also anecdotal evidence that slowdowns are being caused by the ziplines, said Stegman, who points to comments made by travelers and directed to CDOT.

“I’m just wondering who approved the zipline attraction alongside I-70 close to Idaho Springs?” one traveler said June 28. “Not sure if you are aware or not, but the braintrust who pushed it through has single-handedly created a new issue on I-70 going east on Sundays.”

Zipline riders wear harnesses that are attached to pulleys that allow them to ride from tower to tower, or from a tower to the ground.

They have become hugely popular in some areas, including Colorado. Clear Creek County’s first zipline began operating in Dumont on Memorial Day weekend, Stegman said. Colorado Canopy Adventures plans to have the zipline open as long as weather permits.

The zipline course consists of five large wooden towers, all six stories tall, and riders cross Clear Creek several times. The ziplines run parallel to eastbound I-70 and are clearly visible to drivers, Stegman said, adding that there is also rubbernecking among westbound drivers.

The co-owner of the zipline — John Cantamessa — said he’s not sure if the zipline course is causing any significant slowdowns along the highway.

“It could be that they haven’t widened I-70 in 20 years — that could also have something to do with it,” Cantamessa said.

He does know that the zipline complex gets a lot of weary I-70 commuters who notice the course and want to take a ride.

“They want to do something fun, and they get out and stretch their legs and go zipping with us,” Cantamessa said. “I do know we are drawing thousands of people into Clear Creek County who want to go on a little adventure.”

Clear Creek County officials couldn’t be reached for comment.

CDOT faces a dilemma since it has little regulatory power over the zipline businesses, Stegman said. CDOT has say only over the right of way to I-70, she said.

The agency also doesn’t want to stymie the enterprise.

“There has to be a balance between supporting small businesses and economic development and trying to keep mobility, especially on a highway as challenging as I-70,” Stegman said. “We just can’t tell people not to watch the ziplines.”

Monte Whaley: 720-929-0907, mwhaley@denverpost.com or twitter.com/montewhaley