Traffic Signal, Flashers, and Speed Limits
an excerpt from Public Works, January 1990.
Traffic Engineering - Myths and Realities
BENJAMIN E. BURRITT, P.E.
Associate Vice President,
Daniel, Mann, Johnson, & Mendenhall,
Phoenix, Arizona

This publication is presented for "Classroom Use Only."  Its intended use is to stimulate and aid in discussion and role playing within a classroom setting.

Are traffic signals the answer to intersection traffic problems? Let's look at the advantages and disadvantages.

Signals offer maximum control at intersections - they relay messages of both what to do and what not to do. The primary function of any traffic signal is to assign right-of-way to conflicting movements of traffic at an intersection, and it does this by permitting conflicting streams of traffic to share the same intersection by means of time separation.

By alternately assigning right-of-way to various traffic movements, signals provide for the orderly movement of conflicting flows. They may interrupt extremely heavy flows to permit the crossing of minor movements that could not otherwise move safely through the intersection.

When properly timed, the traffic signal increases the traffic handling capacity of an intersection, and when installed under conditions that justify its use, it is a valuable device for improving the safety and efficiency of both pedestrian and vehicular traffic. In particular, signals may reduce certain types of accidents, most notably the angle (broadside) collision.

While many people realize that traffic signals can reduce the number of angle collisions at an intersection, few realize that signals can also cause a significant increase in rear-end collisions.

Normally, traffic engineers are willing to trade off an increase in  rear-end collisions for a decrease in the more severe angle accidents; however, when there is no angle accident problem at an intersection and a signal is not needed for traffic control, there is nothing to trade off and the installation of traffic signals can actually cause a deterioration in the overall safety at the intersection. This situation sometimes prompts the remark, "You mean you won't do anything until somebody gets killed!" What is not fully understood is that traffic signals are not a "cure-all" and that the primary goal of the traffic engineer is to attain the safest and most efficient traffic flow feasible.

In addition to an increase in accident frequency, unjustified traffic signals can also cause excessive delay, disobedience of signals, and diversion of traffic to residential streets.

Traffic signals are more costly than is commonly realized, even though they represent a sound public investment when justified. A modern signal can cost taxpayers between $50,000 and $100,000 to install - depending on the complexity of the intersection and the characteristics of the traffic using it. On top of this, there is a perpetual cost that is almost never considered - the cost of the electrical power consumed in operating a signalized intersection 24 hours a day. This now averages about $1,400 per year.

Speed Limits

One of the most prevalent myths around is that motorists will adjust their speed in response to the numbers posted on speed limit signs regardless of roadway and traffic conditions.

Before-and-after studies consistently demonstrate that there are no significant changes in traffic speeds following the posting of new or revised speed limits. Furthermore, no published research findings have established any direct relationship between posted speed limits and accident frequency, although short-term reductions have resulted from saturation enforcement efforts directed at speed and other traffic law violations. Police agencies necessarily rely on reasonable and well recognized speed laws to control the unreasonable violator whose behavior is clearly out of line with the normal flow of traffic.

Contrary to popular belief, speed in itself is not a major cause of accidents. In fact, there is a consensus of professional opinions that many speed related accidents result from both excessively low and high speeds.

Then why have speed limits? Realistic speed limits - that is, speed limits that reflect the normal actions of the reasonable driver, are useful for several reasons:

• They invite public compliance by conforming to the behavior of the majority.

• They give a clear reminder of reasonable and prudent speeds to non-conforming violators.

• They offer an effective enforcement tool to the police.

• They tend to minimize the public antagonism toward police enforcement that results from obviously unreasonable regulations.

On the other hand, unrealistic speed limits can be detrimental:

• They do not invite voluntary compliance, since they do not reflect the behavior of the majority.

• They make the behavior of the majority unlawful.

• They create public antagonism toward the police, since the police are enforcing a "speed trap."

• They create a bad image for a community in the eyes of tourists.

Flashing Beacons

Do they really cause motorists to reduce their speeds? Flashing beacons (commonly called flashers or flashing lights) are frequently requested by communities in the belief that they will reduce vehicle speeds. Unfortunately, this is not necessarily the case. A flasher is generally installed at an intersection or in conjunction with a warning sign in advance of an area requiring greater than normal care by the average driver. Flashing beacons serve a useful purpose where the flashing yellow is used to alert drivers to unusual conditions that are not readily apparent, such as obstructions in the roadway, uncommon roadway conditions, narrow bridges, or unusual conditions hidden from the motorist's view.

One of the more common locations where a flasher can be used effectively is at a signalized intersection located just beyond a vertical or a horizontal curve, when the intersection is hidden from the view of approaching motorists.

For any flasher to be effective, it must command the respect of the motoring public. In other words, immediately after seeing a flasher, the driver must consistently see an unusual condition that is being singled out for attention. Furthermore, the condition that motorists see must be viewed as serious enough to justify their having been alerted.

When flashers are used improperly and installed at locations where they are not warranted, they soon lose much of their effectiveness. They simply cease to command the respect of the drivers. What happens is that after being continually alerted to a condition which seldom, if ever, appears to be truly unusual, drivers actually stop "seeing" the flasher. When this happens, flashers that are truly needed may well be disregarded by drivers who have become conditioned to believe that flashers are just "window dressing." Because of this normal human reaction, even one improper usage greatly reduces the effectiveness of essential flashers.

Quite often, community requests for flashers are emotional responses to symptoms, rather than attempts to solve underlying problems. To put this into perspective, let's use an appropriate analogy: the case of measles. Obviously, to cure a patient who has measles, the disease itself (measles must be treated - not the symptom (rash). In traffic control, it is not uncommon for public responses to be directed at treating symptoms. For example, in cases where concerned parents are requesting flashers on pedestrian warning signs, a traffic investigation all too frequently reveals that:

• There is no "safe route to school" plan in the community.

• There is no pedestrian safety program in the schools.

• Very young children are allowed to wander to school by whatever route their youthful minds prefer.

• Parents are willing to abdicate their responsibilities by placing the entire burden for pedestrian safety on a traffic control device.

• Local law enforcement officials turn a blind eye to youthful pedestrian traffic violations.

• Where traffic laws are enforced by conscientious law enforcement officials, outraged parents explain away the irresponsible behavior of their children by claiming that the fault lies in inadequate traffic control devices, not in their children.

Flashers that are installed when these conditions exist result in the following:

• The flasher soon becomes part of the normal driving environment and is ignored.

• Parents continue to ignore their responsibilities to their children.

• The community continues to avoid treating the real problem.

• Other flashers, which are justified, are frequently disregarded by motorists conditioned to believe that flashers can be safely disregarded.

In summary, when flashers are properly located, they serve a useful function. When they are used improperly and installed in locations where they are not warranted, they soon lose much, if not all, of their effectiveness. More seriously, improper usage greatly reduces the effectiveness of other flashers installed in areas where there is a real need.

Above all, it is essential to prove that there is a problem which can be solved through the installation of a flasher before actually employing one. Too often, flashers are installed when someone assumes there is, or is going to be, a problem. It is important that flasher installation be minimized to maintain a high degree of respect for the flasher installations that are truly needed.