Everything. In order to evaluate liability, a trial lawyer must be able to figure out your speed, the time it took to arrive at the accident point and the distance you travelled from one point to another. We only need two out of the three elements to figure out the third. For example, if you travelled 100 feet in 10 seconds, we can easily calculate your speed. If you were travelling at 50 miles per hour, and you drove 1/4 mile, we can calculate exactly how long it took for you to travel that distance. Speed, time and distance are crucial in determining liability in your case. What do you think the outcome would be in a case if you didn't exactly know your speed, and you testified that you were travelling at 10 miles per hour, and it took you 2 minutes to travel 100 feet. Obviously, the numbers you recall or estimate are not accurate because they are totally inconsistent with physics. Keep in mind that it's not always crucial for you to know exactly all the details involved in your accident. There are usually other witnesses involved who can add to whatever information you have. Anyway, when your credibility is at stake, you must tell the truth, and understand that while driving you are not looking to calculate speed, time and distance in anticipation of a pending accident or lawsuit.