Effects of Flashing Lights on Traffic

FAQ - Effects of Flashing Lights on Traffic

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. The page on flashing beacons is offered in the interest of broader public understanding of what flashers can do and what factors must be considered before they should be installed.

The content of this page is based on a booklet called "Flashing Lights: Do they Really Slow Traffic?", published by the Arizona Department of Transportation.

What are the effective uses of flashers?

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 motorists' 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, where the intersection is hidden from the view of approaching travelers. For any flasher to be effective, it must command the respect of the traveling 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 the driver sees must be viewed as serious enough to justify having been alerted.

What are the effects of unjustified flashers?

When flashers are used improperly and installed at locations where they are not warranted, they soon lose much of their effectiveness and cease to command the respect of drivers. After continually being alerted to a condition that 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.

With regard to flashers, how do traffic symptoms relate to traffic problems?

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 the case of measles as an appropriate analogy. To cure a patient who has measles, the disease itself (measles) must be treated, not one of the symptoms (e.g., a 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 travel to school by whatever route they prefer.
  • local law-enforcement officials turn a blind eye to pedestrian traffic violations.
  • where traffic laws are enforced by conscientious law-enforcement officials, residents believe that the fault lies in inadequate traffic control devices, not in their or their children's actions.

Flashers that are installed where these conditions exist result in

  • the flasher soon becoming part of the normal driving environment and being ignored.
  • the community continuing not to treat more fundamental problems.
  • other flashers, which are justified, being frequently disregarded by drivers conditioned to believe that flashers can be safely disregarded.


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 that 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 of the utmost importance that flasher installation be held to a minimum to maintain a high degree of respect for the flasher installations that are truly needed.