Similar to high-speed motorways in other countries, autobahns 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 the speed of up to on curves.