Motorcycle Safety Site

Advisory 'SPEED LIMIT' Signs
What they are and what they mean

By: James R. Davis

When a roadway has signage on it that includes the words 'SPEED LIMIT', those signs represent the maximum speed that you may legally drive on that roadway. Despite some lost causes who have tried to argue in court that since it didn't use the word 'Maximum', it could also mean that it was a minimum speed expected, the courts have ruled that these signs represent maximum speed limits.

Did you know that if the words 'SPEED LIMIT' does not appear on those signs then they are NOT speed limits at all? They are known as 'speed advisory signs' and represent the maximum speed at which you are assumed to be able to safely travel on that roadway - that is, with essentially ZERO risk of having an accident caused by that speed. (In other words, as a result of losing traction.)

There may be an exception here or there but in virtually every state across the country you will not be cited for exceeding the speed limit if you are traveling faster than that posted speed unless it is otherwise unsafe to do so (for example: rain, construction, fog, congestion).

I recently had the privilege of explaining this distinction in a court of law to the jury. I was retained as an expert witness in a civil suit involving two motorcyclists who suffered serious injuries while riding at about 3:00 AM one morning on a county road that was posted with a 65 MPH SPEED LIMIT sign. As they approached a relatively tight right turning curve they saw a sign on the roadway that said: "35 M.P.H." - an advisory speed limit sign that warned them about the upcoming turn.

Let me take a tangent here and explain that I was literally shocked by what BOTH attorneys (the defendant's and the plaintiff's) thought those signs meant. The defendant's attorney claimed (outside the presence of the jury) that there was essentially no difference between the two signs I showed above - that both represented a maximum speed limit. The plaintiff's attorney knew differently and was quick to point out the distinction in the courtroom in front of the jury. Unfortunately, he had not the slightest idea what an advisory speed limit sign actually meant and caused trouble for his client by exaggerating his 'understanding' of the distinction in front of that jury.

Both attorneys, by the way, admitted that their clients had been speeding at the time of the accident - at somewhere between 70 and 85 MPH. So this was NOT a case about speeding. It was a civil case about who was liable to pay for the damages resulting from the accident they had.

In any event, let's deal with what those advisory signs mean and why they exist.

Almost 80 years ago, in about 1930, when cars (and motorcycles) were far less capable than they are today, it was recognized that states had an obligation to construct and maintain roadways that were essentially safe to drive upon. It was further recognized that despite the otherwise safe speeds you could drive on those roadways when they were essentially straight lines, some curves simply had to be constructed which could not be driven on safely at the speeds the roadway was otherwise designed to support. So, the states agreed to provide warnings - signs that advised of upcoming danger. These 'caution-warning-danger' signs were invariably associated with tight turns.

But how did they determine what speed to post on those signs? Some turns were tighter than others, after all.

Engineers collaborated with automobile manufacturers and tire manufacturers and determined that automobiles could be expected to safely travel on any road of 'normal quality' provided that they did not experience more than about .2g's of centrifugal force while making a turn. This was well below the traction capabilities of tires at the time. Better rubber compounds and better roadway surfaces since then have made their assumptions even more conservative as time went on.

So, test vehicles equipped with a swinging weight (a metal ball) inside them were constructed with an indicator of the angle that weight made relative to vertical. In other words, if a particular curve was ridden at 35 MPH the weight would be seen to lean, say, 15 degrees away from vertical because of centrifugal force.

It turns out that when the weight angle was between 10 degrees and 14 degrees then the vehicle was experiencing a sideways acceleration of very close to .2g's.

So, advisory speeds were established, rounded to the nearest 5 MPH, at whatever speed caused that weight to lean about 12 degrees. That, it happens, is the same lean angle your motorcycle adopts when riding that curve at that speed.

You've heard it before but I want you to really understand this: You, as the rider of a motorcycle, can ONLY determine its speed and direction of travel. The lean angle you and your motorcycle adopt in a curve is NOT set by you! It is absolutely and entirely determined by your speed and the radius of the turn you are riding. The faster you take that turn, or the tighter that turn is, the greater your lean angle will be. That's the law! (physics).

So those advisory speed limit signs specify a speed that you can virtually guarantee is safe for you to ride that turn at. It is merely a 'caution-warning-danger' signal, not a speed limit. (Note that at least one state finds no difference between a speed limit sign and an advisory speed sign and can successfully ticket you for exceeding the posted speed on that sign - the vast majority of states, however, recognize the difference.)

But if an advisory speed limit is posted at 35 MPH, how fast can you actually ride through that turn with relative safety?

Interesting question. The plaintiff's attorney ruined his credibility in front of the jury by bragging that as a motorcyclist himself he 'knows' that with enough skill and good equipment he could 'easily' handle any curve at twice the speed posted on an advisory - for example, he could easily take the turn in question that was posted with a 35 MPH advisory sign at 70 MPH. Indeed, he said, "I could do that with my car." (He was attempting to discredit me as an expert witness at the time.)

I showed the jury the truth of the matter. (Note that the following models presume that the roadway is flat - has no bank angle. The higher the bank angle, the LOWER the lateral acceleration and, thus, the higher you speed at which you can travel on it without losing traction.)

For example, a curve with a radius of 375 feet taken at 35 MPH:

Notice that the motorcycle would have a lean angle of 12 degrees which means that it would be experiencing centrifugal force (lateral acceleration) of about .22g's. In other words, that roadway would have a 35 MPH advisory sign posted on it even though the rest of the roadway was posted at 65 MPH.

Now see what happens to those numbers when you take that same curve at double the speed - 70 MPH.

The bike lean angle has increased to about 41 degrees which means it is experiencing centrifugal force (lateral acceleration) of about .87g's.

The coefficient of friction of that roadway and the tires on your motorcycle is almost certainly about .8 which means that if you tried that curve at 70 MPH you would have lost traction and washed out before reaching 70 MPH!!!

So, you've heard, just as that attorney stated, that a skillful rider could take any curve at twice the speed posted on an advisory sign and now you know that is NOT TRUE.

If you think that the knowledge, or lack thereof, of those two attorneys is a serious concern given the nature of the trial they were engaged in, then you will be absolutely dumbfounded by what the other expert witness had to say about that curve. She is a very talented motorcycle racer who also happens to be a certified MSF RiderCoach.

In a deposition prior to the trial she was asked if it was possible to safely ride a motorcycle on that curve at speeds greater than 35 MPH. She responded that it was certainly possible to do so. When asked if she could safely ride that curve at 50 MPH, again she responded that she could do so. She added that it was merely a matter of skill and technique.

"Really?" said the attorney. "Then could you safely drive that curve at, say, 100 MPH?" She said that she could do so. "Well, could you safely drive that curve at 150 MPH?" Again, she said that she could.

Her credibility as an expert witness became ZERO. (At 150 MPH her bike would be leaning over at 77 degrees and she would be experiencing about 4.5g's of centrifugal force - well beyond the abilities of ANY motorcycle regardless of skill or technique.)

You can, with almost no risk of losing control, take a curve posted with a 35 MPH advisory sign at 50 MPH assuming no other safety issues. I do it routinely and so, probably, do you. But take heed that listening to an 'old salt' tell you that you could 'safely' handle any curve at twice it's advisory speed will with virtual certainty result in your crashing that bike of yours!

Then there are other things that the 'old salt' will tell you that makes a lot of sense in the right situation but which can also result in your eating asphalt. For example, getting into the habit of always taking curves using a 'late apex' approach. NUTS! By definition, a late apex method involves delaying the start of your actual turn and then using a GREATER lean angle (meaning a tighter radius) than you would use normally at the beginning of the curve and opening it up (lessening the lean angle) as you get to the apex. In other words, using a late apex method while at the same time trying to press speed limits is a test of concepts that is DANGEROUS. Late apex turning is, in my opinion, generally safer than a single lean angle turn, but NOT when getting near speed limits.

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