In common with the rest of Germany, Hamburg offers a predictably efficient and clean public transport network including underground and trams (U-Bahn & S Bahn) to get you around, and because so many of the locals rely on these rather than driving, the city is often light on traffic - meaning that hiring a car in Hamburg is a good experience and gives you a great deal of freedom especially if you're planning to do a lot of driving around in the surrounding area as part of a tour of northern Germany.
