Bonus cars
Look for red circles that appear on various levels. Place your car on the circle until a gurgling sound is heard to unlock a new car. Some of the circles can be found at the following locations:
London
Near the starting point is a ramp on one of the buildings. This leads to the circle on a roof.
Go near the center of the map. The circle is inside the Parliament building (block building with pillars on the outside). Drive through the window to get inside.
Find the strange turn near the Parliament building. Climb up a ramp, then jump to the left or the right, but do not fall off. Turn around to find the circle.
New York
Go near the park above the big tunnel. The circle is on top of a roof that you need to jump to reach.
Go near the aircraft carrier. The circle is inside the second story of a building that you need to jump to enter.