a road warning sign besides a crashed car

Road sign psychology and dangerous driving


Speed Display Signs inform drivers if they are breaking local limits, and are part of road systems internationally.  But how much effect do they have on stopping recurrent speeding? And could changing their use offer a psychological nudge to drivers to slow down?


Drs Smadar Siev and Doron Kliger report on experiments to change speeding behaviours and reduce road traffic accidents.


Read the original paper at: https://doi.org/10.1080/19439962.2019.1682732


Music in this episode is by Scott Holmes



Hello, and welcome to ResearchPod. Thanks for joining us today! In this episode, we’ll be looking at research by Smadar Siev & Doron Kliger, who have been investigating the effect of Speed Display Signs on driver behaviour, in the hope that such evidence can better inform authorities who use these signs as a tool to encourage drivers to stick to a speed limit.


Exceeding the speed limit while driving a motor vehicle is a worldwide phenomenon; we’ve probably all done it to some extent at some time or other. We were running late, we didn’t notice the limit change, and so on. And this isn’t just anecdotal; there is plenty of well-researched evidence to demonstrate just how prevalent breaking the speed limit while driving is around the globe.


A study in 2000 reported that 60% of drivers drove over the speed limit on interurban roads. In 2001 a paper described how 85% of Israeli drivers would speed on country roads when traffic conditions allowed. And in 2012 it emerged that 40% to 50% of Dutch drivers admitted to speeding. According to one statistic, 41 million speeding tickets are issued every year in the United States.


Driving above the permitted speed limit can have tragic consequences though; it has long been identified as having a significant impact on road safety. It is widely accepted that speeding affects road safety in two ways; it both increases the probability of being involved in a crash and the severity of the injuries resulting from such a collision. It has been demonstrated multiple times by researchers that the higher the driving speed, the higher the probability of an accident.


Speed limits are necessary worldwide, as a free choice of speed would lead to more incidents, and in many countries, excessive speed is the number one safety problem. A study in 2018 reported that excessive speed was a contributing factor of one-third of fatal crashes in the United States. In places where speed limits have been reduced, a subsequent reduction in accidents is also seen; a trial in Belgium where speed limits were reduced on certain roads from 90 km/h to 70 km/h resulted in a 5% decrease in accidents and a 33% decrease in accidents involving serious or fatal injuries.


Considering the consequences of speeding, speed limits are critical for road safety; but how is it best to ensure such limits are adhered to? Speed limit enforcement tactics (ie compelling drivers to follow the speed limit through legal compliance) have been met with varying success around the world. Whilst some experiments in speed limit enforcement with cameras, patrols and ticketing have been moderately successful, it is often reported that without continuously increased and highly visible enforcement, driver behaviour reverts back to violation. Upon encountering an enforcement site, for example a speed camera, drivers slow down, but once past the site, they speed up again. An alternative option to enforcement is to try to influence driver behaviour through non-enforcement means such as Speed Display Signs.


Speed Display Signs are electronic signs that display a driver’s speed in real time as they pass the device, and they aim to inform drivers that they are exceeding the speed limit, giving them the opportunity to voluntarily slow down. The assumption is that the effect of the signs will be to both make the drivers aware of their speed, whilst also introducing to them the possibility of speed enforcement in the near future. Past research has shown that like speed cameras these signs often have a short-term effect on driver speed; drivers slow down when they initially encounter the sign but almost always revert to speeding once they are a certain distance away from the device. In one research project, although there was a small overall decrease in speeding when Speed Display Signs were in place, on their removal it took only a week for the average speed to return to pre-sign levels. In another study, 48% of drivers said they would increase their speed if a Speed Display Sign showed them they were below the limit.


Smadar Siev & Doron Kliger, of Ono Academic College, Israel and the University of Haifa, Israel, respectively, have recently been investigating drivers’ traveling speed after exposure to active and inactive Speed Display Signs, delving further into their effects on driver behaviour in a bid to better understand how authorities might use them more effectively to prevent speeding and, therefore, may decrease the number of road traffic accidents. The researchers examined drivers’ speed behaviour in a setting where two Speed Display Signs were located two kilometres apart under two conditions: in Condition I the two SDSs were switched on; in Condition II the first sign was switched off. The signs also had supplementary notices of the speed limit (70 km/h) above and below them. Speed was measured in the vicinity of the second sign, under the two conditions.


It is the potential of being caught by the police that has been identified as the main motivating factor behind speed limit compliance for many drivers.


This has been demonstrated to be the case in several studies , and also In Israel, where Siev and Kliger’s research took place. The results of the experiment showed that in Condition I, where drivers were made aware of their speed by the first sign, they adjusted their speed to the limit at which they thought they were unlikely to be apprehended by the police (in this case around 90 km/h) whether they were, at that time, above or below that limit.


In Condition II, drivers saw the switched off sign and were made aware of the possibility of enforcement but were not informed of their actual speed; the most common reaction then was to slow down to increase the chance of remaining within the perceived limit under which they were unlikely to be apprehended. Siev and Kliger suggest that drivers may have seen the switched off sign and considered they may be approaching a police-enforced ‘speed trap’, resulting in them slowing down to avoid the possibility of being pulled over.


In addition, Under Condition I, with the first sign on, the speed variance, in the vicinity of the second sign, was significantly lower than in condition II, manifesting drivers’ speed convergence around the speed which they thought they were unlikely to be apprehended by the police.  .


What can we learn from this in-depth look into driver behaviour?


Siev and Kliger conclude that their results suggest that drivers do not intend to travel at the speed limit set by the law, but to drive at the maximum speed they can, at which point if they were stopped by the police they would be issued with only a minor penalty. In this case, the goal of reducing speed by the placement of Speed Display Signs was not achieved.


However, comparison of the two conditions shows that whilst speed feedback failed to encourage drivers to slow down, and in fact in some cases seemed to cause drivers to speed up, alerting drivers to a speed issue and suggesting the possibility of apprehension, but without informing them of their speed, resulted in them slowing down.


The authors suggest the value of this research lies in the possibility of employing this tactic as a low-cost means to influence driver behaviour. Authorities could turn Speed Display Signs on and off, suggesting increased or changing enforcement but without the high costs actual enforcement incurs. It appears that there is a real possibility to use the Speed Display Signs in this new way to help reduce road traffic accidents, especially in low resource parts of the world where the effects of road traffic injuries have the direst consequences.


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