The roads are ready. But the traffic forecast is iffy.

The second Highway 40 (Interstate 64) closure, set for Dec. 15, will send up to 75,000 cars a day to streets in St. Louis and the eastern part of St. Louis County, such as Forest Park Parkway, Kingshighway, Manchester Road and McCausland Avenue.

The highway will close between Interstate 170 and Kingshighway and is scheduled to reopen by Dec. 31, 2009.

St. Louis street crews have spent the past two years preparing. They have adjusted traffic signals at more than 100 intersections. They have resurfaced main roads and have a crew of tow trucks ready to move stalled vehicles during rush hours.

Despite those efforts, it's anyone's guess how traffic will flow.

Part of that has to do with the nature of alternative routes east of I-170.

The advantage is that there are more alternative routes to choose from this time, and they're closer together, including Interstates 44 and 70.

The disadvantage is that streets in St. Louis and the eastern part of the county can be more prone to backups. They have fewer lanes (Manchester Road in downtown Maplewood has just two), more cross streets and less distance between traffic signals. These roads run through more densely populated areas and they're more congested during peak times. In some places, such as the intersection of Manchester Road and Kingshighway, bottlenecks occur at rush times almost daily.

"That's why we're recommending that people don't even use those routes and use I-70 and I-44 instead," said Dan Galvin, spokesman for Gateway Constructors, the group of contractors rebuilding 40.

To figure out how to get east and west, transportation officials advise:

— Stick with I-70 and I-44. Whether you're coming from Chesterfield or Illinois, the interstates will work the best.

— Assume that roads closest to 40 will be most congested.

— Have a Plan B and Plan C. If your planned route is Forest Park Parkway, for example, you could find it's packed during morning and evening rush hour.

— Use Interstate 170 as a distributor. It will connect you with several alternative routes, including Delmar Boulevard, Olive Boulevard, Forest Park Parkway and Page Avenue.

— Be flexible with your work hours. Heaviest traffic tends to be 7 to 8 a.m., and 4:30 to 5:30 p.m. Leaving work at 3:30 p.m. or 6:15 p.m. could cut your commute time in half.

Drivers during the second closure also must figure out ways around bridge reconstruction. Hanley Road at Highway 40 closes on Dec. 29 for up to eight months, which will mean more traffic for Brentwood and Big Bend boulevards. Big Bend, however, will close no later than June 1 for up to six months.

NORTH-SOUTH TRAFFIC

Engineers say they are just as concerned with traffic going north and south as they are about east and west.

Kingshighway, for one, is already clogged during morning and evening rush hours between I-44 and Barnes-Jewish Hospital. Traffic on McCausland Avenue increased after the Hampton Avenue bridge closed this fall. Officials expect McCausland to be full during the evening and morning rush once 40's next construction phase starts.

Drivers will need north-south routes to get from I-70 and I-44 to their destinations, and there aren't any easy suggestions. Whether it be Brentwood Boulevard, Vandeventer Avenue, Skinker Boulevard or Big Bend, no one or two routes will work for everyone.

"It's going to be tough," said Linda Wilson, spokeswoman for Missouri Department of Transportation.

St. Louis Street Director Todd Waelterman suggests avoiding roads that are already bad. "If you're a Kingshighway traveler, you might consider going to Vandeventer or Grand."

Here's more advice:

— Consider north-south streets that seem out of the way. Someone living in Rock Hill might be better off using McKnight Road as a partial route to Clayton, if Brentwood Boulevard becomes backed up.

— Have multiple routes and be familiar with them.

— Again, avoid heaviest travel periods. Backups are already forming on I-44 at the Kingshighway exit between 7 and 8 a.m. If that's your preferred route, consider a different drive time.

— If you're using eastbound I-44 to get downtown, don't jump off at Kingshighway to get back onto 40. Take 44 all the way to the Lafayette Street exit.

"There are several other connecting streets between 44 and 64," said Barry Erlandson, traffic maintenance engineer for Gateway Constructors. "We need people to try some of those other ones: Grand, Vandeventer, the Jefferson Avenue bridge. They can get off at Lafayette and take Truman Parkway which becomes 18th Street. Most people can take multiple routes. It's just a matter of getting them to try them."

Two northbound lanes of the Jefferson Avenue bridge between Chouteau Avenue and 40 are scheduled to reopen by Dec. 15. The city and its contractor sped up construction to reopen at least some of the bridge in time for the next highway closure. The southbound lanes will open once construction is finished.

The day of the 40 switch, the Missouri Department of Transportation, St. Louis and St. Louis County will release locations of trouble spots after the morning and evening commutes to help drivers plan. The agencies will continue this until traffic stabilizes. The information will be available on STLtoday.com.

ecrouch@post-dispatch.com | 314-340-8119