United States security agents have closed more roads and highways to bolster security at the venue where Barack Obama is scheduled to speak.

Democratic Party supporters planning to attend the rally at Invesco Field are advised to take free shuttles to and from downtown Denver. Others could use bikes.

The Democratic presidential candidate is expected to address about 75,000 people on Thursday night (Friday 5am, Kenyan time).

Stacey Stegman, the spokeswoman for the Colorado Department of Transportation said, however, some motorists may not have received the new programme regarding road closures on time.

"We are planning for the worst, but hoping for the best," she said of the road closures that were planned by the Secret Service as part of their security plan.

Because of transportation difficulties, some employees in the city opted to work from home during the period of the convention, the biggest event in the city in recent times.

Others adopted schedules that were more flexible.

On the state highways leading into downtown, traffic went down about eight percent.

"The type of traffic we're seeing is what you will see on the weekends," Stegman said.