Category Archives: Safety

Safer urban car lane widths

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Readers need to be aware that this post does have a bias toward giving weaker road users a higher priority for road space. My reading of these design manuals is therefore to identify the widths providing greatest benefit for people aged 8 to 80.
First warning: For this post, lane width is about the distance between painted lines or changes in road surface texture. Kerbs/road bumps greater then 50mm in height limit the available options. See the photo at the end of this post.

New Zealand legislation states that a “lane for the use of vehicular traffic … is at least 2.5 m wide” Reprinted 1 October 2017 – Land Transport (Road User) Rule 2004 (SR 2004/427)

Here is an example (in Schiedam) of narrowed motor vehicle lanes with the cycle lane width being maintained through the intersection. As we can all see, this doesn’t mean excluding heavy traffic (note bus stop). What it does mean is that the safety* a cycle lane width gives can be continued through the intersection.

*“Cycle lanes … provide a modest 10% safety improvement for cyclists, but 30% for pedestrians” This is from the NZTA Pedestrian planning and design guide Table 6.3

Data collected from the Minneapolis-St. Paul and Oakland County – Detroit areas does not support the idea of wider lanes being safer for motorists.

“There is no indication that the use of 3.0- or 3.3-m (10- or 11-ft lanes), rather than 3.6-m (12-ft) lanes, for arterial midblock segments (& arterial intersection approaches) leads to increases in accident frequency.” P23 (P25)

Ref: Relationship of Lane Width to Safety for Urban and Suburban Arterials – 2007 by Ingrid B. Potts, Douglas W. Harwood, and Karen R. Richard

Measures from Tokyo & Toronto also do not support wider lanes being more efficient for motorists. Ref: Narrower Lanes, Safer Streets – Dewan Masud Karim – 2015

Impact on Traffic Capacity and Congestion:

“Contrary to common belief, the results clearly demonstrate that narrower travel lanes, particularly 3.0m lanes, carry the highest traffic volumes (18% higher compared to 3.5m lane) … there is no measurable decrease in urban street capacity when through lane widths are narrowed from 3.7m to 3.0m … Traffic delays on urban roads are principally determined by junctions, not by midblock free flow speeds” p12

Large Vehicles and Narrower Lanes:

“Low volume trucks (less than 5%) experience no operational problems for narrower lane widths … For buses, it suggests using 3.3m for mixed traffic conditions and 3.0m where buffered bicycles lanes exist” p13

Lane widths to avoid – From Handbook for cycle-friendly design by UK based Sustrans 

“Avoid widths between 3.1 and 3.9m” p17

“Bus lanes widths … 3.2m to 3.9m to be avoided” p19

In my opinion it is particularly important that when narrowing lanes that kerbs and speed bumps are kept as low as possible. People stop for a variety of reasons. Bumps can make the ride in a bus uncomfortable or an ambulance trip worse than it needs to be.

Photo Gottingen – Lange Geismar Str

Past Blog post on lanes

A good one way cycle lane width = 2m+

4 lane roads when to

Lane width and cars per hour per lane

Number of cars per lane

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.