It's up to you. In general, it would depend on the situation. Were you following someone else to go somewhere? Or was it something wrong, like driving too closely to the driver in front?
Using having a disability as an excuse isn't really a good plan, especially if it's an excuse for having done something which is dangerous, as you may potentially get yourself into trouble. However, I'm autistic myself, and I typically tell people immediately, since I find that a lack of understanding tends to be what causes a problem since it's classed as a 'hidden disability'.
Hope this helps.