When you think you are not going to make it to the hospital give the ambo's a ring. They are really helpful over the phone and their voice is really reassuring.
DD1 has had two ambulance trips, the first after he was born and the second a very fast trip with lights and siren due to problems that could have happened. Thankfully she made it there in time.
The last one a few months ago was very professional. They were in and out in about 15 minutes but it was all very calm and they never let anyone feel like they were taking over and not considering other peoples feelings.
Another factor they will think about is how far you are away from the hospital you are booked into.