Speed Limit - Regulations - Variable Speed Limits

Variable Speed Limits

In 1965, the first known experiments with variable speed limit signs took place on a 30 km stretch of German motorway A8 between Munich and the border city of Salzburg, Austria. Mechanically variable message signs could display speeds of 60, 80 and 100 km/h, "danger zone" or "accident". Personnel monitored traffic using video technology, and manually controlled the signage. Beginning in the 1970s, additional advanced traffic control systems were put into service. Modern motorway control systems can work without human intervention using various types of sensors to measure traffic flow and weather conditions. In 2009, 1,300 km (810 mi) of German motorways were equipped with such systems.

In the late 1960s, heavily-traveled portions of the New Jersey Turnpike began using variable speed limit signs, in combination with variable message signs. Officials can adjust the speed limit according to weather, traffic conditions, and construction. More typically, variable speed limits are used on remote stretches of highway in the United States in areas with extreme changes in driving conditions. For example, variable limits were introduced in Oct 2010 on a 52-mile stretch of Interstate 80 in Wyoming, replacing the winter season speed reduction from 75 to 65-mph that had been in place since 2008. Similarly, Interstate 90 at Snoqualmie Pass and other mountain passes in Washington State variable speed limits are used to slow traffic in severe winter weather. A response to fog-induced chain-reaction collisions involving 99 vehicles in 1990, a variable speed limit system covering 19 miles (31 km) of Interstate 75 in Tennessee was implemented in fog-prone areas around the Hiwassee River.

A variable speed limit was introduced on part of Britain's M25 motorway in 1995 (on the busiest 14-mile (23 km) section from junction 10 to 16). Initial results suggested savings in journey times, smoother-flowing traffic, as well as a fall in the number of crashes and the scheme was made permanent in 1997. However a 2004 National Audit Organization report noted that the business case was unproved; conditions at the site of the Variable Speed Limits trial were not stable before or during the trial, and the study was deemed neither properly controlled nor reliable. From December 2008 the upgraded section of the M1 between the M25 and Luton will have the facility for variable speed limits. In January 2010 temporary variable speed cameras on the M1 between J25 and J28 were made permanent.

New Zealand introduced variable speed limits in February 2001. The first installation was on the Ngauranga Gorge section of dual carriageway on State Highway 1 with steep terrain, numerous bends, high traffic volumes, and higher than average accident rate. The speed limit is normally 80 km/h.

In 2006, Austria undertook a short term experiment with a variable limit configuration that could increase statutory limits under the most favorable conditions, as well as reduce them. In June, a stretch of motorway was configured with variable speed limits could increase the general Austrian motorway limit of 130 kilometres per hour (81 mph) to 160 kilometres per hour (99 mph). Then Austrian Transport Minister Hubert Gorbach called the experiment "a milestone in European transport policy-despite all predictions to the contrary"; however, the experiment was discontinued.