I also have the same issues Chris, not as a musician, but a frequent traveller for my work. I don't drive most of the time when I'm away, but next week will be doing so in Belgium.
My plan is to do exactly what I do at home and as noblehead said. Test within 30 mins of getting in the car. If >5.0 that's good for the next two hours however many times you get in and out of the car. So if I get up, have breakfast, restest post breakfast and then drive to site, all OK. I then park up for the day, and retest before I leave site.
If I'm at home and at the weekend go into town for some shopping, I'll test after breakfast, go into town, do all I need and will normally be back to the car and home again before the two hours is up. I will not retest under those circumstances