Fair Traffic Laws
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ADVISORY SPEED SIGNS

Every driver knows that they must be careful to enter a curve at a safe speed. It is all too easy to misjudge and enter a curve too fast creating a strong force that threatens to push the vehicle off the road. 

An advisory speed sign may be posted to indicate a safe speed. As noted in the MUTCD, the purpose of advisory signs is to:

"...call attention to unexpected conditions on or adjacent to a highway, street, or private roads open to public travel and to situations that might not be readily apparent to road users. Warning signs alert road users to condtions that might call for a reduction of speed or an action in the interest of safety and efficient traffic operations."

These signs are advisory. Unlike speed limit signs, they do not represent a legal requirement; drivers cannot be ticketed for not heeding the advice given on them.

Advisory speed signs have a great advantage over speed limit signs: Rather than being based on decisions of governmental agencies, the numbers on advisory speed signs are to be based on a law of physics. This is the law stating that a moving object will continue in the same direction unless acted upon by an outside force.

(Reader Note: Following is a description of engineering specifications and considerations for setting advisory speeds. If you are not interested in these details, Click here to skip to CORRECTIVE ACTION NEEDED

 

SETTING ADVISORY SPEEDS SCIENTIFICALLY

A scientific instrument, the ball bank indicator, is available to precisely evaluate the force created when traveling around a curve. A typical instrument produces a reading in degrees from zero degrees to twenty degrees.

A zero reading indicates that no force is present. A five degree reading indicates a barely noticeable force. Ten degrees indicates a force that is apparent to all drivers but is easily tolerated by the great majority. Twenty degrees indicates a force strong enough to be scary to many drivers although under good road conditions it is less than the speed that would cause a vehicle to skid off the roadway .

 The 2009 edition of the MUTCD, Section 2C.08, requires that an engineering study be performed before posting an advisory speed sign and notes that the ball bank indicator may be used to assess the force produced when rounding a curve as noted in below:

Section 2C.08 Advisory Speed Plaque …

Standard:

 

02 … The Advisory Speed plaque shall also be used where an engineering study indicates a need to advise road users of the advisory speed for other roadway conditions.

03 … the Advisory Speed plaque shall carry the message XX MPH. The speed displayed shall be a multiple of 5 mph. ….
06 The advisory speed shall be determined by an engineering study that follows established engineering practices

Support:

07  Among the established engineering practices that are appropriate for the determination of the recommended advisory speed for a horizontal curve are the following:

A. An accelerometer that provides a direct determination of side friction factors

B. A design speed equation

C. A traditional ball-bank indicator using the following criteria:

1. 16 degrees of ball-bank for speeds of 20 mph or less

2. 14 degrees of ball-bank for speeds of 25 to 30 mph

3. 12 degrees of ball-bank for speeds of 35 mph and higher

This specification requires that each advisory speed value determined by an engineering study be rounded to a 5 mph increment, but lacks a rule for the rounding process, which is a serious deficiency. Unless all agencies use the same rounding rule, the problem of unnecessarily large variations between postings may continue.

Based on the specifications for 35 mph or faster, three possible rounding rules are shown in the table below. The approximate range of ball-bank readings that would be produced by each method for advisory speeds of 35 mph or more is also shown.

Rounding Rule

Ball-bank Reading Range

1

Next lower 5 mph value

9-12 Deg

2

Nearest 5 mph value

10-13 Deg

3

Next higher 5 mph value

12-15 Deg









      It is recommended that method 2 be used because the ball-bank values produced would be within the safe, comfortable range for most motorists.

Method 1 would produce values below the maximum comfort level of many motorists and would only partially solve the problem that signs are already generally posted too low.

Method 3 may not be a problem for drivers of automobiles but may be a little high for drivers of large trucks who do not tolerate forces as large as those tolerated by drivers of passenger cars. The Texas Department of Transportation reports that drivers of semi-trailer trucks are uncomfortable at forces associated with ball bank readings larger than ten degrees. 

The final result of employing method 2 would be that most automobile drivers may not find 13 degrees excessive and truck drivers may be able to learn to approach curves at less than the posted value.

Or, a better solution for trucks may be to post separate signs lower than the value established by method 2 and label them for trucks as is presently being done in Texas. See “Guidelines For Dual-Advisory Speed Signing On Freeway-To-Freeway Connectors In Texas”, published July 2007. Click here for pdf copy.

Dual signage may only be necessary for low speeds because ball-bank readings at lower speeds change much more rapidly than at higher speeds. Note in the table below that the ball-bank reading rate of change for curves at speeds less than 30 mph is 1-3 mph for each degree whereas, for speeds of 55 mph or more, ball-bank readings change at less than 1/2 degree per mph.

For example, a 5 mph variation in speed from 55 mph changes the ball-bank reading by only approximately 2 degrees but at 25 mph a 5 mph change in speed would change the ball-bank reading by more than 5 degrees. This may be a major concern when posting advisory speeds for trucks on lower speed curves, such as freeway connector ramps.

Because of the larger rate of change in ball-bank values per mile per hour at speeds lower than 35 mph, it may be advisable to use dual signage for these sharper curves that occur frequently on freeway connector ramps or, because the addition of an additional sign for trucks is a comparatively minor expense, it may be a good idea to post lower speed signs for trucks at all locations.

 

MUTCD SPECIFICATION

Ball-Bank Rate of Change

Ball-bank Degrees

MPH

Degrees

Per MPH

20 mph or less

16 degrees

16

10

3.284

20

1.642

25-30 mph

14 degrees

14

27.5

1.038

35 mph and higher

12 degrees

12

35

0.700

55

0.442

75

0.324

 

 

CORRECTIVE ACTION NEEDED

The amount of force generated at presently posted advisory speeds varies widely from curve to curve. Nearly all readings are less than ten degrees—many are even less than five degrees. 

The difference between forces produced by traveling curves at presently posted speeds is so large that advisory signs are of little use to any motorist. Drivers rely instead on their individual evaluations of the sharpness of curves and other conditions when deciding how fast to drive around them. Most motorists comfortably exceed indicated speeds by significant amounts without endangering themselves or other motorists.

State departments of transportation should recognize that the impact of these problems on the safety of highway travel may be significant. It has been reported that 25% of fatal crashes occur on curves. The rate in Texas is reported at 44%. See Chapter 1, Page 1 of Report No. FHWA/TX-07/0-5439-P1, HORIZONTAL CURVE SIGNING HANDBOOK. Click here for pdf copy.

Now that more realistic standards are established by the 2009 MUTCD, states are further than ever from compliance, which creates more urgency to bring advisory speed signage to current engineering specifications.

 

EXPOSURE TO LEGAL ACTION DUE TO IMPROPERLY POSTED ADVISORY SPEED SIGNS

A potential for lawsuits may exist against states that have not complied with applicable law and engineering standards when posting advisory speed signs.

It is entirely possible that if (or when) deficiencies in posting advisory speed signs becomes known to law firms specializing in traffic accidents, they may pursue legal action against states for accidents where vehicles leave curves because of excessive speed. An argument that warning signs are misleading or an engineering study was not completed before posting a sign may have considerable weight in court.

If the number of fatalities occurring on curves represents a fraction of all fatalities as high as noted above (25%-44%), opportunities for claims may be as many as 8500-15000 per year based on the 33,963 fatalities reported for 2009.

       In the absence of a nation-wide effort, individual states may be well advised to pursue the matter on their own by posting advisory speed values that comply with MUTCD specifications before litigation becomes a problem.

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