Intersection Conflict Warning System Human Factors: Final Report

Figure 29. Illustration. Recommended ICWS signing for minor road approaches to four-lane divided highways.

Minor road intersections with four-lane undivided highways should be signed in a manner consistent with that for intersections with two-lane main roads. For divided highway intersections with narrow medians, engineering judgment should be used in the decision to locate the minor road warnings on the median, overhead, or on the far side of the intersection.

This study did not address sign location on the major road. The results of the Simpson and Troy study suggest that overhead signs at the intersection may not be effective.⁽⁷⁾ Therefore, placing the major road warnings adjacent to upstream shoulders is recommended. In the present study, the activated sign stimuli were placed downstream from a conventional intersection-ahead symbol sign (MUTCD, W2-1) about halfway between the symbol sign and the minor road intersection.⁽¹⁾ Whether both the standard intersection warning and the activated sign are needed was not addressed. The activated sign placement should probably be consistent with the guidance in table 2C-4 of the MUTCD for intersection warning signs.⁽¹⁾

About 70 percent of participants interpreted the active ICWS signs as meaning the same thing with or without the “WHEN FLASHING” placard. Some drivers (up to 50 percent based on the results of this study) interpreted the signs as meaning there was no cross traffic when the ICWS beacons were not flashing. The finding that 28 percent of participants felt that the inactive sign indicated that they did not need to check for cross traffic is of concern. The presence of the “WHEN FLASHING” placard did not seem to be the source of this misunderstanding. Because the ICWS signs were activated, and the intent of the “WHEN FLASHING” placard was to emphasize that point, it is recommended that the placard be used. Education or public outreach may be used to counter the unintended and dangerous conclusion that caution is not necessary when the beacons are inactive. As more than a few participants said, “You should always expect cross traffic.” The activated signs are intended to inform drivers of cross traffic that they might otherwise not detect because of either sight restrictions or perceptual error. The intent is not to inform drivers that they do not need to stop for stop signs or attend to hazards at intersections. A frequent comment in part 1 of this experiment was that the extra caution was needed when the beacons were flashing—this is indeed the logic behind sensor-triggered warnings, and any outreach should emphasize this concept.

The meaning and rationale of blank-out signs for the ICWS application is not easily communicated to drivers. Static signs should be expected to be as effective as blank-out signs at a much lower cost for installation and maintenance. Blank-out signs are not recommended for ICWS applications. Drivers’ misinterpretations of static signs in the event of power failure or sensor malfunction may be addressed with education and outreach.

Recommendations on beacon placement are beyond the scope of this research. However, the MUTCD guidance on beacon placement should be adequate.⁽¹⁾ Where two beacons are used, either to the sides of the warning signs or above and below, alternating beacons should be used to provide greater conspicuity and reduce the possibility of comfort or disability glare.

All of the ICWS sign alternatives shown to participants in this study were diamond shaped. Given the messages recommended here, diamond-shaped signs should be sufficient for all ICWS applications. However, rectangular warning signs with the same messages should be equally effective as diamond-shaped signs.