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Hazard Detection: Speed Variance



The most basic speed law throughout the country is very similar, see below.

PA Basic Speed Law

§ 3361.  Driving vehicle at safe speed.          No person shall drive a vehicle at a speed greater than is       reasonable and prudent under the conditions and having regard to       the actual and potential hazards then existing, nor at a speed       greater than will permit the driver to bring his vehicle to a       stop within the assured clear distance ahead. Consistent with       the foregoing, every person shall drive at a safe and       appropriate speed when approaching and crossing an intersection       or railroad grade crossing, when approaching and going around a       curve, when approaching a hill crest, when traveling upon any       narrow or winding roadway and when special hazards exist with       respect to pedestrians or other traffic or by reason of weather       or highway conditions.

“Reasonable and prudent under the conditions” is pretty much universal. All other posted speed limits should be based on traffic and engineering studies. There are many other factors to consider when setting speed limits, like:

  • crash history
  • grade
  • alignment
  • shoulder condition
  • intersection spacing
  • amount of traffic
  • type of traffic
  • sight distance

These conditions may cause speed limits to be set lower than the 85th percentile speed (explained below). In every event, the lower speed should be posted in 5 mph increments as near to the 85th percentile as possible.

Note: 85th percentile speed is determined by free flow traffic (in other words, cars traveling unrestricted)

On Speeding

In some communities, speed limits are notoriously set lower  because of public pressure to reduce speeding. Pennsylvania allows municipalities to set speed limits at 25 mph without a traffic and engineering study in a residential district that is not a numbered traffic route and is classified as a local highway.

But does this tactic really work? Why do most people exceed the speed limit anyway?

A major issue is when the posted limit is much lower than the 85th percentile speed. In every event, with the exception noted above, traffic and engineering studies should be down to establish safe speed limits. The said truth is that many roadways are posted without traffic and engineering studies and therefore may be improperly posted with very low limits without warrant. This type of speed posting will create many violators and tax police efforts to enforce the law.

We have several residential streets that are posted 15 mph that no one obeys! It’s too low to be taken seriously.

Are slower posted speed limits safer? Not always.

The Plain Truth About Speeding

Have you ever heard of the 85th percentile speed? This is the speed at which 85 percent of  drivers will drive at or below. In other words, the majority of drivers will adjust their speed to what they feel is reasonable and safe. This is commonly referred to as the 85th percentile speed.  Speed limits should be set as close to the 85th percentile as possible, with due regard for other factors (sight distance, grade, alignment, etc.).

Exceeding the posted speed limit, in and of itself, is not the whole story. The variance in speed created by lower speed limits is more likely to create conflicts and hazards.

You can travel at very high speeds in a limited access highway with little potential for conflict because everyone is traveling in the same direction and are going relatively fast also. When does the hazard occur on a limited access highway? When cars are traveling slow. Some limited access highways have minimum speed limits, usually around 40 mph.

Have you ever heard about the 10 mph pace or “pace speed”? This is the highest percentage of vehicles traveling in a 10 mph speed range (i.e. 30 to 40 mph). This method helps determine if the speeds are uniform. The lower the percentage of vehicles traveling within the 10 mph pace the greater the speed variance, which results in decreased safety.

If the posted limit is lower than the 10 mph pace speed, many drivers will ignore the signs and continue to travel at higher speeds. Some drivers will travel at the posted speed limit or close to it.  When vehicles travel at different speeds, this variance in speed causes problems such as tailgating, illegal passing and congestion.  Prolonged periods of time in these conditions can lead to aggressive driving and the increased likelihood of some form of “road rage.”

Posting speed limits at near the 85th percentile actually:

  • Increases safety
  • Makes for easier enforcement –  Blatant speeding is easier to identify
  • Safe drivers are not penalized
  • Police Officers are not wasting time enforcing arbitrary or unrealistic speed limits


Remember, the rider/driver should always make speed adjustments based on the actual road conditions, which change frequently. You should always be able to stop within the distance you can see to be clear or the assured clear distance ahead.

Recognize roadways which have a potential for speed variance and treat this segment of road as a potential hazard. Also recognize roadways that have posted speed limits that appear to low. A particular area of conflict to watch for is multi-lane roads with low-speed limits and vehicle attempting turns or lane changes. The faster over-taking vehicle may be driving faster than his ability to stop AND NOT be perceived within the available sight distance of other vehicles.

Important:  PLEASE note we are not advocating exceeding speed limits in any way, shape or form. We advocate understanding that low-speed limits will increase speed variance and make that segment of road more at risk for crash.

I have investigated more motorcycle fatalities in 25 and 35 mph zones than in any other posted range.

I hope this post gives you a little insight into recognizing speed variance as a hazard.

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