Ahh the logistics of a fast labour are so difficult aren't they! I'm with Stretcher Bearer on this one, I would be focussing on avoiding the inbetween.

To be honest, I don't think I'd want to be labouring in an ambulance, if I could avoid it.
I'd be either driving to hospital at the first sign of labour, if the hospital was no more than about 30mins away. That means I'd need a friend/relative to be staying with us to be there for the kids. Otherwise I'd be staying somewhere closer (like a relative) for the last few weeks if I could. I wouldn't take the car if there was any threat of traffic issues at all. And if the labour hit that hard and fast that I couldn't fathom taking the car, then I wouldn't take the car then either. I'd focus on birthing. You could see if your hospital would let you labour at the hospital, that might help with any early labour 'should we go in or not' decisions, if you know they're happy for you to hop under a shower there and keep labouring, even if you're not too far along yet. Not sure how feasible that is.

If there was traffic, or I felt like things were moving too fast, or I couldn't travel comfortably in the car, then I'd go find somewhere comfy to labour at home and call an ambulance. Sometimes people say 'see how you feel and then call an ambulance', but as Stretcher Bearer's sensibly pointed out, what if something goes wrong? Given that after I accidentally birthed at home, and haemhorraged (ah I can never spell that right), and didn't realise I was bleeding (to be fair, the ambos didn't either, my student midwife friend picked it up), I hate to think how things would have ended up if we'd taken things at a more leisurely pace.
The baby might be fine, but you have you to think of too - the ambulance didn't carry synctocin, any drugs you might need will be at a hospital.

Like I said fast labours are pretty interesting in terms of trying to plan for them! If I had another bub, my plan would definitely involve having someone stay with us for the last period of my pregnancy so I could leave the kids at any moment.

Obviously I haven't got it figured out either, but those are some of my thoughts on the subject. All the best with working through the details.