Fair Traffic Laws
“SPEED LIMIT” SIGNS THAT COULD WORK
In addition to the black-on-white
speed limit signs, which indicate mandatory legal requirements, advisory black-on-yellow
or black-on-orange speed warning signs may be posted.
As noted in the MUTCD, the purpose of these signs is to:
These signs are advisory; their purpose is to provide important information concerning hazards that may not otherwise be noticed by motorists. Unlike speed limit signs, they do not represent a legal requirement. Drivers cannot be ticketed for not heeding the advice given on them. Speed warning signs have a great advantage over speed limit signs: Rather than being based on decisions by governmental bodies, the numbers on warning 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. When driving around a curve, each driver may observe the effect of this
physical law as a “force” that tends to pull their vehicle toward the outside
of the roadway. The strength of this force depends upon the speed of the
vehicle and the sharpness of the curve—higher speeds and sharper curves produce
predictably stronger forces. Drivers who travel too fast around a curve will suffer the consequences as their vehicle spins uncontrollably off the roadway—no exceptions.
Individual drivers have their own tolerance for this force—some motorists feel perfectly safe at speeds that would make others uncomfortable. It is, therefore, impossible to post a number that would specify a suitable speed for each driver who passes.
Nonetheless, warning signs could be very useful to all drivers if all curves were posted with a speed that would create the same outward force on any vehicle traveling the curve at that speed. Motorists who are comfortable with the amount of force generated at the posted speed would travel each curve at the posted speed. If the posted speed creates a force that exceeds their comfort level, they would reduce their speed below the posted value by enough to comfortably drive around the curve. Others may be comfortable driving faster than posted speeds. Thus, each driver would learn by experience how much their speed should be adjusted from the posted speed to avoid exceeding their comfort level. This will work, however, only if drivers experience the same force when traveling around any curve at the posted speed. If they experience different forces at posted speeds, motorists will find advisory signs unreliable and pay little attention to them, which, unfortunately, seems to be the case. Setting Advisory Speeds Scientifically A scientific instrument, the “ball bank indicator”, is available to precisely evaluate the force created when traveling around a curve. This 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 vast majority. Twenty degrees indicates a force strong enough to be scary to many drivers. As with speed limits, the MUTCD requires that an engineering study be performed before posting speed warning signs and advises that the ball bank indicator be used to assess the force produced when rounding a curve as noted below: Section 2C.46 Advisory Speed Plaque (W13-1)
Standard: The Advisory Speed plaque shall be used where an engineering study indicates a need to advise road users of the advisory speed for a condition. … The speed shown shall be a multiple of … 5 mph. Except in emergencies or when the condition is temporary, an Advisory Speed plaque shall not be installed until the advisory speed has been determined by an engineering study. Guidance: Because changes in conditions, such as roadway geometrics, surface characteristics, or sight distance, might affect the advisory speed, each location should be periodically evaluated and the Advisory Speed plaque changed if necessary. Option: The advisory speed may be the 85th-percentile speed of free-flowing traffic, the speed corresponding to a 16-degree ball bank indicator reading, or the speed otherwise determined by an engineering study because of unusual circumstances. Support:: … the 85th-percentile speed on curves approximates a 16-degree reading. This is the speed at which most drivers’ judgment recognizes incipient instability along a ramp or curve. Applying the 85th Percentile Rule As noted in the instructions above, the sixteen degree reading is particularly significant as studies show that the 85th percentile speed of normal traffic rounding a curve corresponds to sixteen degrees. In other words, approximately 85% of drivers are comfortable with the force equal to or lower than the force associated with a sixteen degree reading. This is the reading suggested by the MUTCD as a basis for the speed to be posted on speed warning signs.
This is the present problem: The amount of force
generated at presently posted speeds varies widely from curve to curve. Many readings
are less than ten degrees—some are even less than five degrees. Very few posted
speeds produce a ball bank indicator reading that approaches sixteen degrees. Worse, there are
instances where warning signs are missing for curves where ball bank readings
are greater than sixteen degrees at the posted speed limit.
Not only are posted speeds lower than the speeds deemed safe and comfortable by the the vast majority of drivers, the variation between the forces produced by traveling curves at presently posted speeds is so large that they are of little use to any motorist. Drivers rely instead on their individual evaluations of the sharpness of a curve and other conditions when deciding how fast to drive around it. Consequently, the vast majority of motorists ignores posted advisory
speeds and exceeds indicated warning speeds without endangering themselves or
other motorists. State departments of transportation should correct this situation by performing required engineering studies and posting speed values that produce the same outward force for every curve. Unless there are unusual conditions at a particular curve, the speed associated with a sixteen degree ball bank indicator reading should be used as described in the MUTCD.
It is noted that drivers of large trucks 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 typically
are uncomfortable at forces associated with ball bank readings larger than ten
degrees. See http://ntlsearch.bts.gov/tris/record/tris/00982107.html. To
accommodate this situation, it may be good practice to post dual warning signs
at each curve with the lower speed labeled for trucks.
It is important to note that the MUTCD is an engineering manual that allows for engineering judgment when applying its specifications and recommendations. This allowance is very important as conditions vary from place to place. We must necessarily rely on good engineering judgment to adjust the published recommendations when necessary. This permission must not however, be used as an excuse by government agencies for failure to apply appropriate MUTCD specifications—especially not with respect to advisory speed signs. The critical matter of speed control deserves the best possible effort from all concerned. Unwarranted departures from the MUTCD must not be allowed. Express Your Concerns The following form is provided for your comments concerning any of the information in this web site, or other relevant matters not discussed here. Additional suggestions and comments are invited to help with this project of creating truly fair traffic laws and regulatory practices.
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