DENVER -- The busiest stretch of a major interstate highway will shut down for at least four hours during the last day of the Democratic National Convention while Sen. Barack Obama speaks to 75,000 people at an outdoor stadium adjacent to the freeway.

Interstate 25 from Interstate 70 to 6th Avenue will close starting at approximately 5:30 p.m. on Aug. 28, the Colorado Department of Transportation announced Friday. The highway will reopen when the event and other festivities are over -- although CDOT officials couldn't say when that is. Obama's acceptance speech at Invesco Field at Mile High is expected to conclude at 9 p.m.

Even though Denver Mayor John Hickenlooper promised that downtown Denver will still be "open and accessible," hundreds of thousands of drivers who travel that 5.5 mile portion every day are bracing for the worst.

That stretch normally carries up to 250,000 vehicles a day, making it the most heavily traveled road in the state. I-25 is also the busiest highway in the Rocky Mountain West.

Thursday's morning rush hour will not be affected but the afternoon commute could be drastically altered. Hickenlooper said the closure was a compromised plan agreed on by the city, state and federal governments.

"We are grateful to our local, state and federal partners who shared our commitment to minimize impacts on the local community and to maximize ways for local residents to be a part of this historic event," Hickenlooper said.

"I-25 will be closed for just a short period of time but it's necessary to ensure a safe event for attendees," said CDOT Executive Director Russ George. "While we recognize that motorists traveling through the area will likely experience some delay, we are doing everything possible to keep those delays to a minimum."

Also, from midnight Aug. 23 through midnight Aug. 29, no oversize/overweight vehicles will be allowed to travel on the I-25 corridor between I-70 and 6th Avenue. Freight train traffic in that area will also be halted on Thursday.

Security barricades and fences have already sprung up around Invesco Field. The city said there will be no public parking at the stadium. People with tickets to see the event are encouraged to take public transportation and plan ahead.

Recommended Alternate Routes

To access downtown Denver from southbound I-25:

I-25 to westbound I-70

Exit Pecos Street and go south to 38th Avenue or Park Avenue

Exit Federal Boulevard south to Speer Boulevard

I-25 to eastbound I-70 (recommended)

Exit Washington Street south to Brighton Boulevard

Exit Colorado Boulevard south to 17th or 13th avenues or Colfax

To access downtown Denver from northbound I-25:

Lincoln Street and Santa Fe Drive are the preferred routes

Colorado Boulevard or University Boulevard to 13th Avenue, Colfax Avenue or 17th Avenue

Downing Street

6th Avenue east to Santa Fe Drive or Lincoln Street

To access southbound I-25 from downtown Denver:

Kalamath Street to Santa Fe Drive

Broadway Boulevard

York Street/University Boulevard

Colorado Boulevard

Corona Street/Downing Street

To access northbound I-25 from downtown Denver:

Park Avenue West is the preferred route

20th Street

Brighton Boulevard to I-70

Speer Boulevard (only during designated hours)

For those passing though the city, alternate routes include:

Southbound I-25 alternate routes

I-76 east to I-270 to I-70 to I-225 (preferred)

I-70 east to I-225 (preferred)

E-470 to I-70 or I-25 (tollway)

Northbound I-25 alternate routes

C-470 west to I-70 (preferred)

I-225 to I-70 (preferred)

E-470 to I-70 or I-25 (tollway)

Hampden Avenue west to Sheridan, Wadsworth, Kipling or C-470 to I-70

6th Avenue west to Sheridan, Wadsworth, Kipling or Indiana to I-70

University or Colorado boulevards to I-70