have multiple lanes of traffic in each direction, separated by a central barrier with grade-separated junctions and access restricted to motor vehicles with a top speed greater than . Nearly all exits are to the right; rare left-hand exits result from incomplete interchanges where the "straight-on" leads into the exit. The earliest motorways were flanked by shoulders about in width, constructed of varying materials; right-hand shoulders on many autobahns were later retrofitted to in width when it was realized cars needed the additional space to pull off the autobahn safely. In the postwar years, a thicker asphaltic concrete cross-section with full paved hard shoulders came into general use. The top design speed was approximately in flat country but lower design speeds were used in hilly or mountainous terrain. A flat-country autobahn that was constructed to meet standards during the Nazi period could support speeds of up to on curves. Numbering system The current autobahn numbering system in use in Germany was introduced in 1974. All autobahns are named by using the capital letter A, which simply stands for "Autobahn" followed by a blank and a number (for example A 8). The main autobahns going all across Germany have a single digit number. Shorter autobahns that are of regional importance (e.g. connecting two major cities or regions within Germany) have a double digit number (e.g. A 24, connecting Berlin and Hamburg). The system is as follows: A 10 to A 19 are in eastern Germany (Berlin, Saxony-Anhalt, parts of Saxony and Brandenburg) A 20 to A 29 are in northern and northeastern Germany A 30 to A 39 are in Lower Saxony (northwestern Germany) and Thuringia A 40 to A 49 are in the Rhine-Ruhr to Frankfurt Rhine-Main A 52 to A 59 are in the Lower Rhine region to Cologne A 60 to A 67 are in Rhineland-Palatinate, Saarland, Hesse and northern Baden-Württemberg A 70 to A 73 are in Thuringia, northern Bavaria and parts of Saxony A 81 is in Baden-Württemberg A 90 to A 99 are in (southern) Bavaria A 98 is in Baden-Württemberg There are also some very short autobahns built just for local traffic (e.g. ring roads or the A 555 from Cologne to Bonn) that usually have three digits for numbering. The first digit used is similar to the system above, depending on the region. East–west routes are even-numbered, north–south routes are odd-numbered. The north–south