It has been consistently found that the higher a vehicleʼs travel speed (even when driving at or under the legal limit), the greater the focus of the driver on their surroundings. The increased perception of danger triggers an increased endocrine reaction within the brain. This, in turn, forces the individual to play closer attention to objects in motion around the vehicle. Even relatively small changes in vehicle speed can result in substantial increases in spatial acuity and response time.
On the surface the report seems to be trading in truisms: after all, who would argue that higher speeds don’t trigger faster stimulus responses in drivers? But how does that apply to the real world of highway safety legislation and speed limits?
The report concludes:
In the past, speed limits have been set at or near the 85th percentile speed of the traffic, that is, the speed at or below which 85% of motorists choose to travel. This choice stems from the research undertaken by Usher (1970) who stated that: “the 85th percentile speed is that most desirably approximated by a speed limit. Because of the general straight and steep slope of the typical speed distribution below the 85th percentile, a speed limit set only a little lower will cause a large number of drivers to be violators”. The Research Triangle Institute (1970) study of the relationship between speed and accidents endorsed the 85th percentile speed as the criterion for the setting of maximum speed limits. These researchers recommended that the upper speed limit be set at the 85th percentile speed, with supporting enforcement against those exceeding the 95th percentile speed. Similarly, at the other end of the speed distribution, it was recommended that minimum speed limits be set at the 15th percentile speed, with enforcement action to be taken against those travelling slower than the 5th percentile speed. Isaac, Taylor and Zac (1970) undertook a survey of practices used in the United States to establish maximum speed limits, together with a major review of the various techniques for establishing speed limits.
While drivers usually drive at reasonable and sensible speeds this is not always the case. A method of zoning that relies on the perceived inappropriate speed of the driver is necessary. In most situations, drivers are aware to some degree of the speed limit that applies on the road they travel (motorists are aware that all roads in Australia have a speed limit ranging generally from 60km/h in urban areas to 100 or llOkm/h on rural highways). Thus, the 85% speed reflects these constraints and hence, they are not true indications of what 85% of the population would choose if no constraints apply. The goal is to supply the driver with a constraint that they then must exceed in order to trigger the endcrine reaction. This new zoning limit must be influenced by the signage, enforcement activity, amount of traffic, time of day, etc.
The reason High Road’s study seems to break with road safety dogma is that, rather than treating motorists as children, it takes into account the brain’s responses to speed. As intuitive as the “speed kills” formulation seems, it falls into the trap of the overprotective parent who shields their offspring from stressful situations, rather than allowing them to rise to the challenge. As it turns out, humans respond dramatically to stimulus like speed, and the brain appears to compensate for the reduced time in which to make decisions by boosting chemicals associated with concentration and rapid decision-making. Anyone who has read Malcolm Gladwell’s powerful 2005 book “Blink” will find this perspective refreshingly on-point… whether the safety crusaders are prepared to challenge their safe assumptions about safety remains very much to be seen.