[Sherlock doesn't seem too concerned as he puts another crisp into his mouth. He'd prefer stealing a car, actually, but he knows Joan is more concerned with rule-following than he is. Not to mention his current physical state might make things difficult.]

Between a half and three-quarters hour before we need to stop, then.

[He turns his attention onto Joan for a moment.]

I'm already a fugitive. I'll hotwire a car if you can find a suitable car park to go through. And, you may as well play hostage if we're seen.