when to go the hospital based on distance

Im planning on asking my doctor but the question is in me head right now so I thought I'd ask some mommies  
So we live a hour and a half away from the nearest hospital that delivers babies 30 minutes of that drive is gravel roads and I know that most hospitals don't really want to see you in till your contractions are 5 minutes apart but when we live this far out when should leave? When they are 15 minutes apart? More less?