Category Archives: Safety

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.

Development Contributions (DC) higher density traffic bonus

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Looking at the statement in the Outline of significant changes to the Development Contribution Policy 2018/19 – Page 5

“Under the existing policy some high-density developments received a discount for placing lesser demand on the Council’s services – but in fact placed demand equivalent to those of larger dwellings”

Note the key word change here is “high-density” in the proposed DC policy reports, whereas it was “higher density” in previous DC policy. When it comes to traffic in Hamilton, increasing (higher) density cannot be said to place equivalent demands on roading infrastructure as lower density areas. The graph below shows the wider Nawton area population trend line increasing at a steeper angle than the local traffic trends line.

What is being suggested in the new DC policy is that the location of a suburban stand-alone 3-bedroom dwelling on a 650m2 section places an equivalent demand on services as a 3-bedroom dwelling in a higher-density urban area. This is where the definition of “high-density” may be clouding assumptions. It is possible that high-density (such as High Rise) has greater costs than suburban density, but urban density is debatable.

The traditional traffic assumption is that a new suburban “3-bedroom dwelling equals 10 traffic movements per day”. The proposed 2018 DC policy (p42) for 2-bedroom and 1-bedroom dwellings uses “Residential Conversion Factors – Two Bedroom – Factor = 0.689 & One Bedroom – Factor = 0.477” (p41), which rounds up to 7 and 5 traffic movements per day respectively.

This next graph measures, as percentages, the wider Hamilton East area population growth vs traffic actual vs traffic predicted.

The above graph includes the actual and predicted Ruakura/Peachgrove intersection counts. We should note that the development of Hilda Ross retirement village occurred around 2002, quickly followed by Ruakura/Peachgrove/Wairere road building. Once the construction work force left this area, traffic movements evaporated to a level below that in 2002, despite the 200-plus dwellings having been built at Hilda Ross village for about 450 people (census mesh block 0896002 & 0896402). Yet at its gate is an oversized road designed to cater for predicted phantom trips (see below for predicted traffic), which by design has a negative/deadly impact on safety for weaker road users in the area.

To Summarise: the wider Hamilton east area’s density increased from around 1,900 people per km2 (ppkm2) in 2001 to 2,200 ppkm2 in 2013. For Hilda Ross village, which is dominated by smaller dwellings, there is a population density of over 5,000 ppkm2. However, a negative effect on traffic movements appears to be measureable. Like the assumptions used to predict the 80,000 vpd for 2026, the 0.477 factor for 1 bedroom homes is an assumption, not a fact.

For more on one-bedroom dwellings, Hamilton City Library has Kol Peterson’s book Backdoor Revolution: The Definitive Guide to [Accessory Dwelling Unit] ADU Development

The following excerpt is from Chapter 9: Impacts on a City

“Space efficiency and location efficiency – In general … neighbourhoods that are walk-able, bike-able, or transit oriented … Residents in such neighbourhoods will tend to have dramatically fewer vehicle miles travelled in a year than US residents living elsewhere.”

Reference: Ruakura Peachgrove Noise AECOM Nov 2009 – page 84 – Table 9: Predicted 2026 Upgraded Traffic Volumes