The central reservation in the United Kingdom and other densely populated European countries (where it is known by their local names) is usually no wider than a single lane of traffic. In some cases, however, it is extended. For instance, if the road is running through hilly terrain, the carriageways may have to be built on different levels of the slope. An example of this is on the M5 motorway as it climbs up the side of the Gordano Valley south of Bristol. In Birmingham and many other cities, suburban dual carriageways may have trees or cycle lanes in the middle as a wide central reservation.