Practice: Herbert sets the pace

Driving the Champion Racing Audi R8 prototype, Herbert turned a lap of 1m04.310s (92.981mph) on the 1.661-mile temporary racing circuit located adjacent to RFK Stadium. Sunday's two-hour, 45-minute race will be the first racing event held in the US capital city in more than 80 years.

"I think they've done a marvelous job with this circuit," said Herbert, who will co-drive on Sunday with Stefan Johansson. "It's very smooth and wide, and there are many places for overtaking. I think the spectators will see an interesting race on Sunday."

The factory Audi R8 of Frank Biela and Emanuele Pirro was second fastest in Friday's practice at 1m04.529s, followed by the factory Audi R8 of Rinaldo Capello and Tom Kristensen at 1m04.578s. Close behind the Audis was the Panoz LMP-1 of David Brabham and Jan Magnussen at 1m04.831s.

The GTS class for production-based cars was led by the Saleen S7R of Terry Borcheller and Franz Konrad, turning a lap of 1m10.460s. The two factory Corvette team cars were second and third, with the top three separated by only a few tenths of a second.

Leading the GT class was the Alex Job Racing Porsche 911 GT3 RS co-driven by Lucas Luhr and Sascha Maassen, turning a 1m13.607s. The runner-up in Friday's practice was the Racer's Group Porsche of Kevin Buckler and Brian Cunningham.