The field of cars in the Defense Advanced Research Projects Agency (Darpa) Urban Challenge is down to just 11 entrants.

At the start of the week 35 teams were ready to compete for $3.5m in prize money for running a robot-controlled car around a series of tests in an urban landscape.

After a series of tests earlier this week, the number was cut to 21 and now just 11 teams remain.

"The teams were subjected to a series of rigorous tests to determine whether they were equipped to compete in the Urban Challenge Final Event," said Darpa director Dr Tony Tether.

"Finalists were selected based on performance in the tests as measured by data collected and evaluated by trained scorers in the testing areas.

"The trials tested the vehicles' ability to merge into traffic, navigate four-way intersections, respond to blocked roads, pass oncoming cars on narrow roads, and keep up with traffic on two-lane and four-lane roads."

The course has also had to be slightly redesigned to allow one of the teams, TerraMax, to use its chosen vehicle, a 15-ton dump truck. So far the truck has outperformed other vehicles in some tests.

Saturday's Urban Challenge Final Event at the former George Air Force Base in Victorville, California is expected to be attended by thousands.