The stretch of the Pan American Highway from Dominical to San Jose is beautiful but the drive is not for everyone. The road is well paved and smooth the whole way, but curves and winds its way up the steep mountains (not good for people with motion sickness). From Dominical to San Isidro del General, it’s fine, but after San Isidro, you have to watch out for fog and occasional washouts. Because of the altitude, the clouds roll in in the afternoon, making visibility tough so avoid driving at night. Also be sure to check that the road is open before venturing out- there are sometimes landslides that temporarily close it for repairs. Even with all that though, if you’re up for some adventure, it is an awesome, scenic drive and one that we’ve done many times. The coastal route along Highway 34 is faster though and a much easier drive so most people go that way to get back to the airport. We have always tended to hit traffic in Cartago going the Interamericana route.  
 
As for hitting traffic in San Jose, traffic is somewhat constant. Times to especially avoid though are right before and during big holidays like Semana Santa and Christmas, and weekdays during am and pm rush hour.