IDAHO SPRINGS — With a bang — literally, a boom and a cloud of dust — major rock blasting began Friday on the Twin Tunnels project, signaling the start of major improvements to the traffic-choked Interstate 70 mountain corridor.

Local, state and federal officials pushed down an old-time ceremonial dynamite plunger to set off the explosive charges at the east-bound bore of the tunnels. There has been some preliminary blast work around the tunnel, but now workers can start “turn-under” work inside the structure, said officials.

Idaho Springs Mayor Jack Morgan said it’s about time. Communities along the corridor have waited for decades for improvements to the interstate.

“There will be short-term pain for people traveling on I-70” as regular blasting continues through much of the summer, he said. “But there will be long-term gains because of this project.”

The $106 million project includes adding an eastbound lane from the east Idaho Springs interchange to U.S. 6 as well as the expansion of the eastbound tunnel to include three lanes and shoulders.

The work also calls for improving lighting in the tunnel and widening the entrance to encourage eastbound drivers to keep driving into the tunnel without braking, Colorado Department of Transportation planners say.

Improving the Twin Tunnels, built in 1961, is the first of several projects designed to improve safety and mobility along the interstate, CDOT executive director Don Hunt said.

“When this highway was built, there were 1.8 million people in Colorado,” Hunt said. “Now we’re at 5.2 million people, but the highway has stayed the same. Something has to be done.”

Expansion along the corridor will reduce accidents by an estimated 35 percent and save Sunday travelers about $11.4 million in travel time and fuel costs, Hunt said.

Among the first things necessary to widen the eastbound tunnel is large drilling equipment to create holes in the hillside. Explosives are then packed into the cavities for controlled, localized blasts that remove large sections of rock, CDOT said.

During each blast, I-70 traffic will be stopped in both directions just before the tunnels. Eastbound traffic will be stopped for up to 20 minutes and westbound traffic for up to 30 minutes. This will give crews time to conduct the blast, clear out any materials and inspect the westbound tunnel bore before allowing traffic to proceed.

The project contractor — Kraemer-Obayashi — is using two sets of drilling equipment to speed up the tunnel blasting and widening.

Blasting should be complete by midsummer, and eastbound I-70 traffic probably will be moving through the new tunnel by the end of the year, CDOT spokeswoman Amy Ford said.

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