Taking the Politics Out of Speed Limit Setting—a Member Responds: NMA Newsletter #729


By Don Bain, NMA Oregon Member 

Editor’s Note: Don submitted these comments to the NMA National Office in response to Newsletter #724 Taking the Politics Out of Speed Limit Setting.

While sound from an engineering perspective, there’s an inherent challenge with speed surveys and the 85% percentile speed point: establishing and documenting what “free-flowing conditions” are, as a defined standard, and whether the definition and its context are met at and during the data collection period.

For example, over what period–day, night, during specific time windows, holidays/work days, how long is the speed representative of what?

Would it be during the presence or absence of in-road temporary activity (construction, parked delivery trucks, a metal plate over a pit, pedestrian crossings, etc.) and off-road distractions (LED billboards, pedestrians, costumed-people twirling signage, etc.), the weather conditions, etc.?

Lane and shoulder widths vary; some segments are curvy or straight, with better or poorer sight lines and good or poor signage. All of the above affect speed distribution.

The free-flowing condition distribution will be lower at a place with a narrow roadway and narrow shoulders driving in foggy morning rush hours with a lot of roadside activity. It would be higher at a straight and wide street with no roadside culture and great lighting at 5:00 am. It’s common sense such variables occur far more or less at different locations along the same road.

The situation produced is easy to game or hack–choose how, where, and when data are gathered to produce the desired result, and then apply it for a long time to long stretches of roads with many varying conditions. Or don’t bother (no time or budget) and merely deploy surveys when/where/how long it’s convenient—garbage in, garbage out.

None of the above accounts for what’s safe behavior for a particular driver and vehicle.

A driver traveling at the 95% percentile may be perfectly safe, and someone driving at the 40% percentile might be unsafe at that speed. A 95% instance may have occurred during much better conditions of either vehicle, driver, weather, distractions, etc. The 95% vehicle may handle great, be well maintained, have excellent visibility, with great tires, and be a skilled driver in good physical condition. The unsafe 40% vehicle might be an old wallowing barge, poorly maintained, have poor visibility, and driven by someone with poor eyesight, slow reaction time, or limited skills. Not to mention the use of in-vehicle distractions. What’s safe for one driver to do can be unsafe for another.

A one-size-fits-all speed limit is crude at best and substitutes drivers’ real-time location awareness and control for a flawed occasional metric.

It’s also subject to political whims, distorted budgets, enforcement types, gaming, and unjustified shaming, and it can criminalize safe behavior while neglecting unsafe behavior.

Yes, this is an argument for ditching numerical speed limits altogether–the vast majority of individual drivers are better at determining & controlling what’s safe behavior in real-time and place, which is the fundamental basis for setting a numerical limit via survey.

Most drivers favor their real-time/place judgment over the number on a limit sign. That’s likely why there’s a distribution of surveyed speeds.

It’s better to merely have the basic rule and safety enforcement based on actual conditions/situations within an envelope of reasonableness as determined by the objective facts of the instance.

Yes, this would be based on the discretion of drivers, officers, and the courts, which long has been ubiquitous with numerical limits and enforcement of other traffic laws. The MUTCD already has unevenly deployed standards for signage which give fair warning to drivers about conditions affecting safe driving. While tickets yield revenue, signs cost money, mixing justice and money is another can of worms.

The ability of third parties to accurately monitor and micromanage driving behavior matures into the ability to give social scores & grant privileges accordingly (see China, it’s coming here). Automated issuance of tickets based on literally every single and otherwise safe infraction increases. The above issues will come into sharp focus because the strictly deployed will yield nearly everyone multiple tickets daily.

As a practical and political matter, discretion cannot be ditched.

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