Speed Design Vs Posted Speed

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The point of this post is to show a weak link between design speed and posted speed, and to illustrate that the optimum posted speed limit is set more by leaders than engineers.

A 5 km/h speed sign is what one sees when entering the car-park at Te Rapa PlaceMakers, while entering Bunnings car-park the speed is assumed to be 50 km/h. They are similar car-park designs, types of business and client types.

A 10 km/h speed limit is used at all supermarket car-parks unless you’re shopping at Countdown

15 km/h is the speed at Countdown supermarkets, unless you shop at Countdown Liverpool St, which is 10km/h. There are no engineering, design, franchise or user differences. What makes things different is the managers’ thinking about the level of risk and what is a ‘reasonably practicable’ speed to reduce the risk of harm.

20km/h is a benchmark speed. The AustRoads safety team, which led the Grey St safety improvement plan, mentions that speeds of around 20 km/h are used to avoid serious injury. A 20km/h limit is used in the German city of Koblenz for its city centre streets. The road use in central Koblenz isn’t different to Hamilton’s Barton St (30km/h) or Victoria St (50km/h between Bryce St and London St); only the risk of serious harm from speed is different.

30km/h is now what 50km/h was last century. Most urban areas in Europe use 30km/h as a default speed. There is a good reason: “for a speed choice of 30 km/h instead of 50 km/h, the estimated reduction in fatal crash risk is 95%” (from the AustRoads safety team that led the Grey St safety improvement plan). Even the Chief Executive of AA advocates for 30km/h speed limits. In Seattle, the speed limit on all residential (non-arterial) streets is 20 mph [30km/h], and the speed limit on arterials is 25 mph [40km/h], unless otherwise posted. Speed is the critical factor in the severity and frequency of crashes. Learn more about why Seattle lowered speed limits in 2016.

A 40km/h [25 mph] default speed is used increasingly across cities in the USA. Speed is the key to reducing the severity of harm resulting from mistakes. For Example: “Since 2014, [in] New York City … the city’s traffic fatalities have declined 28 percent with a 45 percent decline in pedestrian fatalities — bucking national fatality trends, which have increased 15 percent over the same period.”

50km/h is a misused speed in urban areas. It is seen to be an efficient speed for journeys that involve moving a very large number of motor vehicles along a single lane. The problem is that cities are made up of places, where people walk from homes or car-parks to socialise face-to-face, to shop, to go to school, or many other day-to-day activities, all of which involve crossing roads as pedestrians. A pedestrian will be almost always seriously harmed, and in fact has a 50% chance of being killed in a conflict with a person driving a car travelling at 50km/h. Never mind who makes the mistake, the pedestrian is always the victim.

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