Speed Limits and SSTs – Holistic Road Safety
Humans have an SST, a Standard Stimulus Threshold. This threshold is different for everyone and differs for the same person according to their situation. This makes arbitrary speed limits, speed limits which do not take into consideration the end user, a potential danger to all road users.
What is the SST?
The Standard Stimulus Threshold is related to the mental stimulation we are receiving at any given time. If the physical stimulation from our surroundings is sufficient but not too great we enter the the “zen” state. We have no spare mental capacity to process any more stimulus from our surroundings and we do not have a deficit of stimulus from our surroundings which would cause us to start thinking about things other than what is happening immediately in front of us.
Obviously, this zen state is where we operate at our peak, whether it be professional sports or driving. Performance is optimised when all of our mental capacity is focussed on the job-at-hand.
Speed Limits
Speed limits can force the driver to operate the vehicle in a manner where the immediate stimulation from the surroundings is below what is required to keep that driver’s attention from wandering.
In most cases the speed limit is too low although it can be also be too high if weather is bad, congestion is high or if something unusual is occurring. But generally speed limits will be too conservative.
The Effect of Speed Limits
When drivers are forced to operate their vehicles at a speed which is below their SST, at that specific time and circumstance, the driver will subconsciously look to fill the void in stimulation in order to reach their SST. The void can be filled with many things but the most popular must be daydreaming. A state where the driver is thinking about things other than what is happening in front of them. Obviously a negative for safety.
Physically the driver may adjust settings in the car eg cruise control, radio station, ac temperature, seat position, radio volume, eat, make a hands free call, look at surroundings other than the road and so on. Whatever it requires to reach their SST at that time. All a negative for safety.
Solutions.
As road planners we need to help keep drivers operating as close to their SST as possible and in a way that is directed solely on the act of driving.
Driver Education
A driver may be travelling at a speed they think is appropriate but isn’t due to lack of experience and/or imagination. We should always be considering the worst case scenario as we are driving and have a plan to deal with that worse case scenario should it occur. When a driver has a good awareness of all the possibilities, the speed at which the SST is met for driving is lowered.
Appropriate Speed Limits
In some cases the speed limit can be woefully inaccurate for the speed the traffic would naturally flow at. In these circumstances, to refocus drivers, we have a number of options.
Road Markings
Removal of road markings to enhance the feeling of speed and enhance uncertainty with regards to the motions of other drivers. Making rules less well defined forces drivers to pay more attention to those around them, making their SST achievable at lower speeds.
Lanes can be made narrower, lanes can be made less straight and so on. The answer will depend on countless factors but generally anytime where a question mark can be inserted into the mind of a driver will lower the speed at which the SST is achieved. This approach can be taken in steps until the desired speed is achieved. As complacency is reduced so is the speed at which the SST can be achieved.
And the most obvious, if the above means are felt as not appropriate or required, the speed limit can be reassessed. The 85% rule is one method but this may need to re-visited after the speed limit has been adjusted. Drivers may drive at higher speed after the speed limit is increased. Repeating the 85% rule until we get to a speed limit where the most drivers are as focussed as possible on driving.
This process is not perfect as we could still have up to 15% of drivers operating below their SST for the speed limit.
Accident Rate
The vast majority of accidents occur to inattention. The road user knowingly or unknowingly has exceed their SST by a large enough margin to be involved in an accident. Other road users maybe be caught up in this accident. It is tempting to reduce speed limits on road where the accident rate is higher however given what we know about our SST and the fundamental nature of accidents, the problem may be exasperated.
Before conclusions are jumped to, the type of accident needs to be established. Is there a type that stands out above others, for example accidents involving pedestrians. If so, what is leading to pedestrians being involved in accidents. Changing the speed limit of vehicles will not change the behaviour of pedestrians and if limits are lowered, could even make pedestrians more complacent. The solution needs get the SST of all road users to be met by the road. Do we have crossings which lead pedestrians to not look sufficiently. Is the view of road users obstructed. The answer will rarely be one thing.
In Conclusion
Ultimately we are trying to get everybody to their destination in the fastest and safest way possible. To achieve safety we need all road users to be concentrating on their interaction with the road and on as little else as possible. Forcing all road users to think (which can be helped massively with education) will see everyone being more aware and travelling at appropriate speeds. The temptation to use speed limits as the magic bullet, without altering the nature of the road or educating road users invites situations which could be more unsafe.
It would be nice if lower speed limits were the golden bullet but this ignores the human element and ultimately that is who are using the roads and who we are trying to protect.
Ignoring the user of a system when designing the system will probably lead to a less than optimal system.
