The Red Light Running Crisis
Is it Intentional?

Office of the Majority Leader
U.S. House of Representatives
Red Light Running Scam
May 2001

3. The Theory: If There's a Problem, Lengthen the Yellow

The Report
1. Something Funny
2. Camera Revenue
3. The Theory
4. The Fact
5. Code Changes
6. Cameras Ineffective
7. Conclusion
8. References

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(200k, PDF format)

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There is no doubt that red light cameras present an attractive option for those interested in collecting additional revenue. But there may be another way to solve the red light running "crisis" -- lengthening yellow times.

A little yellow makes a lot of difference

A case study of two intersections entitled "The Influence of the Time Duration of Yellow Traffic Signals on Driver Response" (1980), reported that a 30 percent increase in yellow time yielded substantial safety benefits. "The Results in Table 3 show that the extension of yellow duration reduced the frequency of potential conflicts in all cases studied," (page 27).

The first site studied found an extra second and a fraction of yellow had an immediate and definitive safety pay-off: "An increase of 1.4 seconds or about 30 percent in yellow duration virtually eliminated all potential conflicts at the Maryland site," (page 27, emphasis added).

Similarly, the second site in Georgia realized a 75 percent reduction in potential conflicts following a 32 percent increase in yellow time. These figures agree with those found in Section 4 of this report, below.

The yellow light's purpose

To understand why an increase in yellow has such a significant safety impact, one must consider the traditional purpose of the yellow traffic light. The yellow indication is designed to warn a motorist approaching an intersection that the signal is about to turn red. The yellow light should be long enough for the approaching motorist to either, (a) come to a safe stop before the intersection, or (b) continue clear through the intersection before the red light appears.

An inadequate yellow time will either prevent motorists from coming to a safe stop or force them to enter the intersection on a red light. Neither option should be considered acceptable.

The Dilemma Zone The diagram illustrates what happens when an automobile approaching an intersection sees the yellow light. Drivers who are in the "Can't Go" zone as the light turns yellow know they are too far back and won't be able to reach the intersection before the light turns red -- they must stop. Drivers who are in the "Can't Stop" zone know they're too close to the intersection to stop safely -- they must proceed. But when the yellow time is inadequate, there is place in between both zones where the driver can neither proceed safely, nor stop safely. Engineers call this the "Dilemma Zone."

A properly timed signal will have enough yellow time that driver's will never be faced with the impossible choice presented by the dilemma zone. By determining the stopping and clearing distances for a given approach speed, one can always calculate a safe yellow time that offers drivers a safe option, by design, every time.

What if there's a problem?

Still, an engineering formula may not perfectly account for all the variables that might exist at an intersection. In such cases, the engineer has a tool, known as a countermeasure, that he must employ to remedy the situation. Namely, the engineer must lengthen the amount of yellow time. Even the 1985 ITE proposed recommended practice provides for this yellow time "measure of effectiveness":
When the percent of vehicles that are last through the intersection which enter on red exceeds that which is locally acceptable (many agencies use a value of one to three percent), the yellow interval should be lengthened until the percentage conforms to local standards. (Page 6.)

It is the duty of an engineer to double-check his work and make sure that there is not a problem with red-light entries at each intersection.

Red Light Camera proponents agree

This truth is not disputed. Even in literature intended to promote the use of red light one finds the inescapable truth that lengthening the yellow can be the appropriate thing to do if there's a problem. In the study "Red Light Running and Sensible Countermeasures", author Richard Retting agrees that longer yellow times can often substantially reduce accidents and red-light running:
Signals that provide insufficient yellow intervals cause some drivers to run red lights inadvertently. However, many drivers who run red lights are provided adequate opportunity to stop safely but choose instead to proceed through a red light signalů. (Page 1.)

Increases in the length of the yellow signal toward values associated with the ITE-proposed recommended practice significantly decreased the chance of red-light running. (Page 2.)

What is surprising is that the author, despite acknowledging that insufficient yellow causes red light running, considers red light cameras as the only solution. In the first citation given above, he makes an effort to blame motorists for running lights, even when the yellow time is inadequate. Furthermore, in the report's "summary and conclusion" (page 4), "signal modification" rates only a passing mention in half of a sentence -- he devotes the rest of the discussion to the virtues of red light cameras. Note that the signal modification he refers to is the already shortened ITE practice, not the longer yellow times a properly timed intersection would use (see Section 5, below).
'Longer yellow signals reduce red light running, there is no question about it,' said Richard Retting, senior transportation engineer at the Insurance Institute for Highway Safety. 'I can't say with any certainty if that has any effect on crashes, but there is some evidence that longer yellows can cut down on crashes.' (Las Vegas Review Journal, October 20, 2000.)

Eighty percent of entries occur during the first second of red

Red Light Entries
at Site 1
Red Light Entries
at Site 2
Hourly Average Hourly Avg.
(after one second of red)
Hourly Average Hourly Avg.
(after one second of red)
5.6 1.2 1.3 0.3
79% entered on first
second of red
77% entered on first
second of red
The relation between yellow time and red light running is most clearly found in the Insurance Institute for Highway Safety's study of red light running entitled "Red Light Running and Sensible Countermeasures" (1998). Although the report's intention is to prove the need for red light camera enforcement, the data in the report provides additional insight into the red light running question. A chart found on page 2 of the report (summarized below) indicates quite clearly that almost 80 percent of red light entries occur within the first second of the red light indication.

This strongly suggests that inadequate yellow time is the major cause of red-light entries. If the vast majority of red light entries occur in the first second after the yellow light expires, it is reasonable to assume an additional second of yellow time on that light will yield a nearly 80 percent decrease in red light entries.

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