motoring enthusiasts and looked at with a mix of awe and terror by outsiders." Some speed limits are implemented on different autobahns. Certain limits are imposed on some classes of vehicles: Additionally, speed limits are posted at most on- and off-ramps and interchanges and other danger points like sections under construction or in need of repair. Where no general limit exists, the advisory speed limit is , referred to in German as the Richtgeschwindigkeit. The advisory speed is not enforceable; however, being involved in an accident driving at higher speeds can lead to the driver being deemed at least partially responsible due to "increased operating danger" (Erhöhte Betriebsgefahr). The Federal Road Research Institute (Bundesanstalt für Straßenwesen) solicited information about speed regulations on autobahns from the sixteen States and reported the following, comparing the years 2006 and 2008: Except at construction sites, the general speed limits, where they apply, are usually between and ; construction sites usually have a speed limit of but the limit may be as low as . In rare cases, sections may have limits of , or on one ramp . Certain stretches have lower speed limits during wet weather. Some areas have a speed limit of in order to reduce noise pollution during overnight hours (usually 10 pm – 6 am) or because of increased traffic during daytime (6 am – 8 pm). Some limits were imposed to reduce pollution and noise. Limits can also be temporarily put into place through dynamic traffic guidance systems that display the corresponding message. More than half of the total length of the German autobahn network has no speed limit, about one third has a permanent limit, and the remaining parts have a temporary or conditional limit. Some cars with very powerful engines can reach speeds of well over . Major German car manufacturers, except Porsche, follow a gentlemen's agreement by electronically limiting the top speeds of their cars—with the exception of some top of the range models or engines—to . These limiters can be deactivated, so speeds up to might arise on the German autobahn, but due to other traffic, such speeds are generally not attainable except during certain times like between 10 p.m. and 6 a.m. or on Sundays (when truck drivers have to rest by law). Furthermore, there are certain autobahn sections which are known for having light traffic, making such speeds attainable during most days (especially