my boyfriend's screaming, stop, stop, stop! i said, i'm trying, i'm trying! i went to press the brake and i pressed the accelerator because my foot was caught and we sped up. we hit the car in front with quite some impact. >> reporter: exactly how does the wrong shoe affect the way you drive? >> this is our test kit. >> reporter: at this crash center in derby, we tested a whole array of footwear, measuring that critical fraction of a second it takes to brake. in this stationary test vehicle, we simulated driving at 50 miles an hour. trainers came out best. it took just under half a second to hit the brake. stilettos were a bit slower, taking .52 seconds. flip-flops fared significantly worse at .60. wellington boots were a slow .66 seconds. wedges came out bottom at .69 seconds. the only thing in the highway code on shoes is not to wear anything that prevents you using the controls.