If you're a short run to the hospital leave it as long as you can....go by how you feel rather than how far apart your contractions are because that can be really unreliable in some women.

If you have a one hour drive across town in peak hour it pays to plan a bit I think....I've done a few long hospital drives with women in heavy labour and it's not really optimal. Aside from the car not being a great place to labour comfort wise it literally drags you out of your zone too and out of that nice rhythm that most women get into labouring at home.