Hospital is 45 mins away...when should I go?

Mel
I'm a FTM at 39 weeks and 2 days. I've been having more regular contractions but have had trouble timing them because it's hard to figure out where they start and stop...sometimes they seem to go on for a while, other times they go away as soon as I move around. I don't think I've lost my mucus plug and my waters haven't broken. 
My doctor recommends that I labor at home as long as possible but my hospital is 45 minutes away. Since we don't have a car, we will have to take a taxi. I know that every labor is different but I'm worried I will wait too long or not really know when to go...
My questions is..at what point would you go to the hospital?
Thanks~