California Code, Vehicle Code - VEH § 26456

Stopping distance requirement tests shall be conducted on a substantially level, dry, smooth, hard-surfaced road that is free from loose material and where the grade does not exceed plus or minus 1 percent.  Stopping distance shall be measured from the instant brake controls are moved and from an initial speed of approximately 20 miles per hour.  No test of brake performance shall be made upon a highway at a speed in excess of 25 miles per hour.