Road Management Journal
Road Management Journal
Copyright © 1997 by TranSafety, Inc.
November 1, 1997
TranSafety, Inc.
(360) 683-6276
Fax: (360) 335-6402
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How Effective Are Cross Traffic Warning Signs?

Cross traffic control devices at intersections warn drivers that not all approaching vehicles are required to stop. Sometimes drivers on the "minor" roadway assume that drivers must also stop on the "major" roadway. While a number of factors may cause the resulting "right-angle" collisions, crash data show that some locations reported reduced accident frequency after installing cross traffic warning signs. Other locations reported no significant change. These signs are not standardized and are used inconsistently. Increasing the use of cross traffic signs may cause drivers to expect them and to assume that in their absence all traffic must stop. The research suggested that the signs would be helpful in locations where driver perceptions are a major factor in crashes and/or where rights-of-way have changed.

J. L. Gattis reported his research on cross traffic control devices in "Cross Traffic Signing for Stop Signs." The report appeared in the Transportation Research Board's Transportation Research Record No. 1553, Traffic Control Devices, Visibility, and Evaluations, published in 1996.

BACKGROUND

The Manual on Uniform Traffic Control Devices (MUTCD) does not incorporate cross traffic signs. Because no standards exist for the message, color, shape, and placement of these signs, the author considered the following in his research:

  1. The present use of 'Cross Traffic Does Not Stop' and similar supplemental traffic control devices,
  2. The circumstances under which these traffic control devices (TCDs) have been installed,
  3. The effectiveness of such TCDs, and
  4. The broader issue of where to go from here--does the current state of affairs suggest alternative solutions, new warning methods, a change in the MUTCD, or new research directions?

Various factors related to the following can cause operational problems at intersections:

Drivers' previous experiences may affect the way they react at an intersection. They may expect all approaches will stop at an intersection where all roadways are consistent in width, traffic volume, number of lanes, pavement, or roadside development.

CROSS TRAFFIC LITERATURE

A 1993 report listed seven types of signs warning that traffic from one or more directions does not stop. One survey showed that the preferred sign was a black 'Caution' on a yellow background with an additional 'Cross Traffic Does Not Stop' on a white background. Figure 1 shows four examples of cross traffic warning signs.

FIGURE 1
Examples of Cross-Traffic Signs


Figure 1-a


Figure 1-b


Figure 1-c


Figure 1-d

PROJECT METHODOLOGY

The researchers mailed a survey to federal, state, provincial, and local traffic engineering agencies in the U.S. and Canada.

RESPONSES TO INITIAL EXTENT-OF-USE-SURVEY

Transportation professionals provided "a wide range" of opinions on cross traffic TCDs. The cross traffic TCDs surveyed included supplemental plaques mounted on stop sign poles, intersection beacons, and stop sign beacons.

Comments from Those Opposing Signs

Some of those surveyed objected to the use of cross traffic signs, saying that:

Some agencies objected to cross-traffic signs because they felt that, because such signs are nonstandard, their use could cause liability problems.

RESPONSES TO SUBSEQUENT DETAILED SURVEY

The researchers conducted a telephone survey of some respondents. This resulted in more detailed information.

Reasons for Use of Cross Traffic Signs

The survey determined three categories of reasons for use of cross traffic signs:

  1. a change in the assignment of right-of-way, such as conversion from an all-way to a two-way stop, or a change in the direction that is required to stop for a former two-way stop,
  2. unexpected intersection characteristics, such as an uphill approach, a former T-intersection that had been converted to a four-way because of a new road opening, or a railroad grade crossing on one of the approaches, and
  3. driver behavior, including wrong impressions of the requirement for other approaches to stop or ignoring of the stop signs.

Crash Experiences Associated with Use of Cross Traffic Signs

The information collected provided some crash details. Data supplied from surveys in Arkansas showed that drivers may not be seeing approaching vehicles. Data from Florida suggested that crashes decreased when an unnecessary traffic signal was changed to a two-way stop. A Michigan county reported little difference in crash rates after cross traffic warning signs were installed.

In Indiana, it appeared that a four-way stop or signaled intersection reduced crashes better than did cross traffic signs. Since traffic volume increased during the time of the study, however, the researcher assumed that this contributed to the higher number of crashes at the cross traffic intersections.

A report from Oregon showed a dramatic improvement after cross traffic signs were installed. Because the data were collected for a longer time before the installation than after, this study may need to be continued.

In Saskatchewan, it appeared that cross traffic signs had reduced crashes at an intersection of two highways. Data from a Wisconsin county showed some increase in the crash rate at one intersection and a slight decrease at another.

EFFECTS OF CROSS TRAFFIC SIGNS ON CRASHES

The research compared crash data for a variety of signs both before and after sign installation.

Specific Effects on Crashes

Most of the data came from intersections where signs had been placed to correct driver right-of-way misconceptions or violations, rather than from intersections where the right-of-way was changed. The surveys revealed inconsistent information on the effectiveness of cross traffic signs at the intersections studied. By way of accounting for these inconsistencies, the author offered:

The issue of regression to the mean must be considered. The tendency for a high accident period to be followed by a low accident period can create the illusion that a particular accident countermeasure is effective, when the decline is really the result of chance. A common traffic engineering practice has been to obtain 3 years of before and 3 years of after period data to overcome the effects of small sample size, among other reasons. Certainly there is more stability in 3 years of data than in 1 year of data. The Wisconsin data set showed an amount of stability over successive 3-year periods. In contract, the Michigan data exhibited running 3-year totals of all accidents in the before period ranging from 11 to 27 (3.7 to 9 acc/year). Even the 4-year running totals yielded rates ranging from 5.2 to 7.5 acc/year. In this case, even a 3-year period does not appear to give a true average accident rate.

Inferences from Crash Data

The data showed that the "right-angle accident problem" at stop-controlled intersections is more than one problem.

  1. Door posts, "intersection skew," or other vehicles may block drivers' line-of- sight.
  2. Drivers may be failing to see "plainly visible oncoming through-roadway vehicles," partly due to visual clutter in the background.
  3. Drivers may be assuming that cross traffic is supposed to stop when unusual rights-of-way "violate driver expectancy."

The study showed that when crashes resulted from several factors, corrections to right-of-way assumptions may prevent only some problems. Determining exactly what caused improved crash rates was difficult because:

 a reduction in the number of right-angle intersection crashes might not be the result of an increase in safety specifically due to cross traffic controls, and  the agencies surveyed provided data with differing amounts of detail.

The researcher suggested that future studies of the effect of cross traffic control devices should be based on individual accident reports. Such reports provide the detail needed to categorize the different types of right-angle intersection collisions.

POSSIBLE FUTURE DIRECTION

Suggestions for traffic engineers to achieve uniformity included:

  1. Do not use cross traffic signs.
  2. Use all-way supplemental plates at all applicable stop-controlled intersections.
  3. Increase the size of the all-way supplemental plates in use.
  4. Install cross traffic warning signs only at "problem intersections."
  5. Install cross traffic warning signs at all intersections where one or more directions of traffic do not stop.
  6. Use a special stop sign when cross traffic does not stop--for example, a stop sign with through arrows from right and left (see Figure 1-d).

Copyright © 1997 by TranSafety, Inc.



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