Driving tests

Are wider roads safer, and how are road widths decided?

A two-lane road in New Zealand usually consists of a lane in either direction that is 3.25m wide plus a 0.5m shoulder on either side which can be increased to 1.2m if it’s designed for cyclists. Traffic signs are located at least 0.6m from the road edge. If there’s a flush median marked, this will be at least 1.5m, and the road lanes can narrow to a minimum of 2.5m, as mandated by Traffic Regulations 1976 (the maximum width of a vehicle in New Zealand without an oversized load permit). A 6.5m minimum combined lane-width provides for two maximum legal width vehicles (2.5m wide body + 0.24m wide wing mirrors) to pass whilst staying within their lane.

State Highways are treated differently and their widths are in the table below.

Vehicles per day Lane characteristics
Lane width Shoulder width Gravel strip Total width
>4000* >3.5m >1.5m >0.5m <12m
2000-4000 3.5m 1.5m 0.5m 10m
500-2000 3.5m 0.75m 0.5m 8.5m
<500 3.3m 0.2m 0.5m 7m

*If funding allows, the road width can be up to 12m.

Roads are not uniformly wide, though. Corners are wider as drivers use more of the road’s width when turning (this especially applies to long vehicles), and more leeway is given for minor errors such as misjudging how tight or sharp the corner is.

Does making lanes wider reduce the risk of accidents?

It would seem like common sense that if you make lanes wider you increase the separation between vehicles and therefore would reduce the chance of a head-on collision if a driver drifts within the lane.

According to an American study NCHRP Report 330 Effective Utilization of Street Width on Urban Arterials, Transportation Research Board, Washington, D.C., 1990, “Narrower lane widths (less than 11 ft [3.35m]) can be used effectively in urban arterial street improvement projects where the additional space can be used to relieve traffic congestion or address specific accident patterns.” This research was in relation to America having lanes that are 3.6m wide.

The Midwest Research Center reported, “A safety evaluation of lane widths for arterial roadway segments found no indication, except in limited cases, that the use of narrower lanes increases crash frequencies. The lane width effects in the analyses conducted were generally either not statistically significant or indicated that narrower lanes were associated with lower rather than higher crash frequencies. There were limited exceptions to this general finding.” In their study, a narrower road resulted in less casualties.

How do drivers react to different lane widths

On motorways and expressways drivers expect to encounter a consistent environment with traffic flowing at relatively high speeds in the same direction, some kind of protection from oncoming traffic and not much to run into on the sides of the motorways. Drivers feel safe because motorways are engineered to accommodate driving that is much faster than our speed limit (100kph). Drivers are artificially restrained to what has been defined as a ‘safe speed’ because if there are no speed limits the perception of what is a safe speed can exceed 200kph, as proven by the Autbahn in Germany where some stretches have no speed limit.

Conversely, in urban areas, particularly busy, built-up areas, there are vehicles parked on the sides of the roads, trees and sign posts close to the kerb, pedestrians crossing the road, roundabouts, side streets with vehicles turning in and out, buses stopping and pulling away from bus stops, narrower lanes due to space constraints, more service covers for water and power which make the road surface less even, more visual clutter with signage, advertising, shop frontages, etc; and more variety in road markings. In short, it can be a rapidly changing and stimulating driving environment where drivers have to be much more aware of potential dangers. This automatically causes drivers to drive more slowly.

Making the road wider increases the time it takes to cross the lane for pedestrians, and for vehicles turning right from the main carriageway, or from a side road onto the main carriageway, while simultaneously giving drivers the confidence to drive faster on that road. Narrower roads cause drivers to drive more slowly and carefully, according to the research.

Does making lanes narrow reduce their capacity?

Research in America found that once lane widths drop below 10 feet (3.05m) that their capacity begins to fall. Drivers will slow down naturally, and may have to slow down to overtake cyclists or overtake parked vehicles. With wider lane widths, drivers tend to speed up, according to this research from Texas Transportion Institute.

Transit gives the following figures in its State Highway Geometric Design manual.

Clearance to fixed obstacle close to road Lane Capacity (% of 3.5m lane capacity)
3.5m lane 3.3m lane 3.0m lane 2.7m lane
1.8 100 93 84 70
1.2 92 85 77 65
0.6 81 75 68 57
0.0 70 65 58 49

What are the advantages of narrow lanes?

Narrower lanes take up less room leaving more room for street beautification (trees, artwork, etc), pavement for pedestrians and lanes for cyclists (both of which reduce traffic congestion. Narrower lanes are cheaper to produce and maintain as there is less tarmac to build and service and, in the case of cuttings or other works required to support the road, less work is needed. There is less runoff of rainwater as more space can be left as vegetation. If private land has to be purchased for the road, less land has to be purchased.

Lanes above 3.6m wide encourage drivers to use them as two lanes, leading to an increase in sideswipe crashes whereas repeated studies, like this one, have indicated that lane widths below 3.6m do not increase crash risk except in very specific circumstances.

Lane widths

Minimum lane width: 2.5m

Minimum State Highway lane width: 3.3m

Minimum combined lane width: 6.5m

Minimum shoulder: 0.6m

Minimum shoulder if used as a cycle lane: 1.2m

Minimum flush median width: 1.5m

Maximum total width: 12m

Usual total with for State Highways: 7m, 8.5m or 10m.

driver training

Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild

Tagged with: ,
Posted in Advice
When was the last time you checked your Road Code knowledge?

Try some tests for free!

Road Code car quiz

Road Code motorbike quiz

Road Code heavy vehicle quiz