If you had a rough estimate on time-of-day you could make an educated guess for speed based on traffic patterns. Assuming the bad guys wished to blend in with the traffic to avoid suspicious behavior that would potentially lead to being pulled over by a police officer. Even then, it's only one variable in the equation which is easily verifiable in a location with only a few bridges where some of them are eliminated almost immediately before even getting in the car.

But I don't recall them covering anything on that subject in the movie.