[Sherlock had really wanted to avoid bringing Joan into this, but she's right. They're both in danger now and the only thing they can do is stay safe until they can eliminate the danger. In this case, it means running and hiding until he gathers his strength and they both gather intel.]

You didn't stop for petrol on the way to the motel, so we should have a few hours. You might want to get some sleep.

[She'd be the one driving, obviously. It makes more sense for her to sleep at the motel when he could easily sleep in the car.]