At what point in pregnancy is it not safe to travel an average length of travel 2.5 hours?
It depends. If you're on the road and you pass through several towns (that is, civilization), you can pretty much travel up to term. For plane travel, I would stop at about 32-34 weeks. And nowhere in pregnancy should you trap yourself into a situation where you can't get to a hospital if you need to--i.e., overnight camping trips, long cruises. Flights over oceans should be carefully considered if you're high risk. The absolute safest time to travel is second trimester, because you're out of miscarriage territory and not yet into preterm labor territory.