In this post I want to talk about a speed as a hazard, particularly speed variance. Motorcyclists should learn the effects of posted speed limits have on the motoring public and when these situations present a hazard.
Years ago I was asked to speak to a private community association group. What was their number one complaint, speeding! The crowd was getting hostile about too! Then I asked, “who in the room has never broken the speed limit?” One lady answered and quickly said, “me…but I don’t drive!”
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
Conclusion
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.
Around here the 85th percentile is usually 5 MPH over the posted speed limit.
That’s good. With speed limits posted so close to the 85th percentile there should be less conflict. Many crashes there?
Thank you for the overview and explanation of rates of travel and establishing speed limits. I certainly can appreciate how community pressure can mess up posting reasonable speeds on roads. One country road near here is posted at 25mph because of a quarry and related trucks. But the road would easily handle much faster speeds and people routinely travel faster.
Speed variance is an issue for me on my Vespa, especially on limited access highways. Even though the scooter will run along at 70mph that is often too slow.
Thanks again for a great site.
Steve Williams
Scooter in the Sticks
Very well said. Thank you for posting this most excellent explanation.
Most people around here in Oregon do 10 miles over the limit as they know they won’t be pulled over for it. On I-5 and some other major highways you will just about get run over if you only do the speed limit or 5-10 miles over. Definitiely a hazard when on a motorcycle and we try to avoid those roads when possible even when in the car.
In PA, no person may be convicted upon evidence obtained through the use of radar unless the speed recorded is 6 or more over the speed limit.
For electronic devices which measure speed by calculating elapsed time between two points by sensors, in an area where the legal limit is less than 55 mph, if the speed recorded is less than ten miles per hour in excess of the legal speed limit, no person may be convicted.
Municipal police in PA can ONLY use the latter as the statute only allows members of the Pennsylvania State Police to use radar. Which means they have to be doing 10 mph or above in zones less than 55 mph.
Many officers allow an additional 5 mph buffer before they issue a citation. So in many areas of PA, you have to be exceeding the speed limit by 15 mph before you attract attention (but can be written for 10 over the limit). Also, many officers recognize speed limits that are posted to low and stick with the 15 mph over before they cite.
I’m curious about the term “limit” as it relates to speed limits. I always thought the number on the sign was the maximum speed under ideal conditions. Night, rain, snow and heavy traffic would mitigate ideal and the limit would be lower. The driving too fast for conditions thing.
Is this true or merely another misconception I carry along with me.
And since this is a motorcycle blog, what are the legal guidelines for not being on the road on two wheels?
Thanks!
steve
Steve,
Yes, the maximum speed limit is always predicated on ideal conditions. Speed limits which are posted pursuant to a traffic and engineering study have many factors incorporated into the final recommended speed limit.
Regarding safety, speed limits should be adhered to.
Regarding prosecution, the benefit always goes to the driver. Thus, the enforcement tolerances!
More crashes occur because of driving too fast for conditions (no stated speed) than exceeding posted limits. Although more violations occur with posted speed limits because it is easily quantifiable.
The basic speed law calls for prudence, difficult to prosecute because can be subjective, especially if there is no collision.
Posted limits are just a number, if you exceed the enforcement tolerance and have been timed by a an approved state device, then easy to prosecute.
As stated, ideally, the posted limit should be as close to the 85th percentile speed as possible. You may be surprised how many roads are posted without a supporting traffic and engineering study. When this occurs speed variances increase and safety decreases.
As far as legal guidelines for not being on the road on two wheels, I don’t really understand your question. I am not aware of any prohibitions for motorcycles. If you are referring to another class of vehicle I need you to be specific.
Does this answer help?
I interact quite frequently with motor cops in Oregon. Not because of infractions, mind you, but because I am a trainer. One thing they’ve been telling me is that the basic rule has become very subjective to both define and prosecute. It is still applicable outside municipalities but inside city limits the posted speed limit rules.
Of course, there is still room for citations like reckless, careless, etc. inside the cities even if the driver is within the posted limit.
As for me, my motto is “Ride Prudently!”
Rueben,
Thanks for the information on speed limits. Your answer suggests prudence on the road and that is something I try to do. It does lead to the last part of my question that was unclear.
When I am riding home from work in the snow on my Vespa am I actually allowed to do that or do the road conditions imply some legal trouble ahead? I have found myself on the road more than once when the weather changed to snow. I have Heidenau SnowTex tires but that is more for the enhanced stickiness on dry roads when the temperatures plunge.
thanks,
steve
Very refreshing to hear this subject presented as sensibly as you have done Ruben. Officers of the law can hardly approve of breaking speed limits! But recognising that some people do, indeed most of us if we are honest, is important to road safety (and that unrealistic limits can be set by local governments). UK police officers are taught that attitudes to speeding are very important and to assess their own attitude to speed. With low speed limits it can be very hard to retain a defensive riding position if everyone around you is ignoring those limits. Recently the speed limit was dropped to 20mph (from 30) inside Oxford’s city limits. Motorcycle riding colleagues noted the temptation to ignore this limit as they felt unsafe being constantly overtaken by cars (and buses!) continuing to travel at 30. I had a similar experience riding in Virginia a couple years ago – being overtaken by local car drivers in a 55 limit whilst riding an FJR1300. I felt extremely vulnerable!
Steve,
I don’t know of any law that would prohibit you from riding in the snow. You know the risks involved on two wheels. Adverse weather is a problem for four or more wheels too. I don’t believe the issue of prudence applies to the type of vehicle you drive/ride. Sometime it is prudent just to stay home…lol.
I have been caught in the snow before and I can tell you that I returned to headquarters and parked as soon as possible. I believe the risk far outweigh the benefits of riding at this point and it no longer enjoyable or practical.