The science of road signs

Research by Associate Professor Sam Charlton of the University of Waikato found that drivers in a sign awareness test only noticed an average of 15.3% of the speed signs on the roadside. And these were adequately maintained signs, not the overgrown or vandalised ones.

Dr Charlton says instead of relying on signs, most drivers unconsciously adjust their speed based on perceptual cues like road width and roadside objects. One of Dr Charlton’s favourite examples is Fitzgerald Glade on SH5 between Tirau and Rotorua in the Waikato. There, the bush encloses the road and gives drivers the sensation they are travelling faster than they actually are, with the effect that most drivers reduce their speed by 10 to 15%. The opposite happens at an overtaking lane just a few kilometres later; the lanes are wide with very few objects on the side of the road, and drivers unconsciously speed up. These speed changes are automatic, based on the look and feel of the road, and most often happen on roads that are familiar.

Dr Charlton uses the term ‘self-explaining roads’ to refer to roads that provide a series of perceptual cues and clues as to how they should be driven. And, sometimes, the cues and clues about how a road should be driven are at odds with the actual risk. This is especially true at corners where drivers are usually well under the speed limit, but can still be driving too fast for the corner or the conditions, and crash.

Sometimes the cues drivers use are contradicted by speed signs. Dr Charlton uses a presentation which shows five near-identical roads with differing speed limits, ranging from 50km/h to 100km/h. If drivers fail to notice the speed sign or if they are driving ‘on autopilot’, there is nothing about the look of the roads to help them drive at the correct speed. Dr Charlton believes that changing the look of the roads so that they ‘explain’ themselves, using a range of perceptual methods, would have a significant impact on drivers’ speed and be far more effective than changing signs.

Speed limit signs are also important to most motorists because they would rather not get speeding tickets, particularly if they are caught out by speed limit changes that they haven’t noticed. Last year, Police issued 919,639 tickets, including 627,943 speed camera tickets.

Speed limits are administered by the NZTA, but district councils keep a schedule of all speed limits within their jurisdiction.  All speed limit bylaws are under review – a process due for completion by October this year – which is likely to result in changes to some posted limits.

So, what constitutes a reasonable speed? Some officials would like to see all but SH1 speed limits reduced to 90km/h and urban speed limits reduced to 40km/h.

Two-thirds of surveyed AA Members are opposed to the lower urban speed and three-quarters are opposed to the lower highway speed. The same survey found most Members drive the open road at slightly over 100km/h; half overtake at no more than 110km/h; 89% overtake at no more than 120km/h.

School zones are a particular point of conflict. Parents want traffic to slow down when children are arriving at or leaving school but, in law, there is no such thing as a school zone. Schools only need to be signalled 160m from their grounds if they are on a 100km/h road. In urban areas it is optional. And, although the point at which that optional sign should be is 65m, Police consider school zones to be 250m either side of a school’s gates. AA Members strongly support (89%) stricter policing of school zones, but only if they are signposted.

The absence of speed camera signs is a particular annoyance to AA Motoring Affairs General Manager Mike Noon. He points out that in most countries the areas where speed cameras operate are heavily signed in order to reduce speeds. In his view, a speed camera that is issuing a lot of tickets indicates there is something wrong with the message reaching, or not reaching, drivers.

Ultimately, it comes down to psychology. The crudest form of behaviour modification is punishment. Tickets work to some extent, but they don’t work on everyone.

The best way to change behaviour is to change environments. There has been a tendency for traffic engineers to dominate the process of designing the roading environment, even though graphic communication and perceptual psychology are not part of their training. While engineers and Police most certainly have a role in defining the roading environment, perhaps it is time a wider range of expertise was encouraged, to provide drivers with the best possible guidance on the road.