Sacramento commuters and traffic officials face perhaps the most worrisome and uncertain summer ever.

What's going to happen when state highway officials post "closed for repair" signs on busy Interstate 5 in downtown Sacramento next month?

Will there be freeway chaos regionwide? Will gridlock rule downtown streets?

Or will drivers adapt – take a train, a bike or a back road – and manage to flow smoothly through the June and July construction period?

State Department of Transportation officials are hopeful but unsure. They've never closed a core Sacramento freeway before for repairs.

"We're on pins and needles," Caltrans spokesman Mark Dinger said. "Your guess is as good as ours."

Although the series of closures starts on May 30 – a Friday night – the real test will come Monday, June 2, when tens of thousands of commuters return to work downtown.

They will find I-5 northbound closed in downtown as crews begin the two-month process of digging up and replacing the decayed and leaking roadway.

The northbound lanes will get two closures this summer. Southbound lanes downtown separately will experience two closure periods.

Dinger said it could well get ugly, illustrated by this unsettling scenario:

If you live in Elk Grove and need to get to the Sacramento airport during that first Monday morning commute, your 17-mile drive may be a two-hour nightmare.

With northbound I-5 closed, airport-bound drivers will have to trail-blaze a new route, possibly joining hordes of truckers detouring onto West Sacramento freeways, or braving the often-clogged Capital City Freeway.

Downtown commuters, meanwhile, are likely to shift to other freeways or surface streets.

The result, Caltrans engineers say, will be a ripple effect of congestion throughout the metropolitan area.

Caltrans' computer simulations suggest freeway drivers could experience two to three times normal congestion.

Two sections of freeway are likely to be especially hard-hit.

Interstate 80, coming toward downtown from Davis and West Sacramento, could be trouble. It is designated as a truck detour route during the closure. Every additional truck is equal to three or four more cars on the road.

Officials also warn drivers to steer clear of southbound I-5 in Natomas during the weeks those lanes are closed in downtown.

Drivers will be able to reach downtown on I-5 those weeks, but traffic will be funneled into two lanes and forced to take the Richards Boulevard and J Street offramps. That means backups as far north as Natomas.

Those are what Caltrans calls "worst-case" scenarios – what happens if...

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