City crews started work Sunday evening to get rid of the expanded bike lanes they installed on Folsom Street in July.
After complaints about traffic jams, the city decided to turn Folsom back into a four lane road between Canyon Boulevard and Spruce Street.
The city said protected bicycle lanes on Folsom Street south of Valmont Road to Pine Street will remain in place, as will the buffered bike lanes south of Arapahoe Avenue to Taft Drive.
In July, Boulder took a lane in each direction on Folsom to expand bike lanes on both sides of the road, and add a buffer zone between bikes and traffic.
It was followed by outrage from drivers who said traffic on the street was a nightmare with only one lane in each direction, especially during rush hour.
"The times I've taken it, it's probably doubled the time it took to go through that route," said driver Mike Yuhas.
Cyclists staged a protest on Folsom earlier in the week arguing that extra space on the road makes a safer ride for everyone.
"I think the more room you have, even if it's just a little bit, I think it's really helpful," said Hallye Webb.
The city said work to change Folsom back to four lanes for drivers will take two weeks.
"Some work might require temporary lane closures, and drivers may want to consider taking alternate routes until the project is complete," said city spokeswoman Sarah Huntley in a press release."No daytime closures are expected."
The smaller bike lanes that have traditionally been on Folsom Street will remain.