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Publication Number:  FHWA-HRT-13-044    Date:  August 2013
Publication Number: FHWA-HRT-13-044
Date: August 2013


Traffic Control Device Conspicuity


There is no generally accepted measure of the conspicuity of TCDs. Eye tracking has been used to assess whether TCDs receive glances, but the research presented in this report indicates that some TCDs were noticed and remembered even though they did not receive an eye glance. In addition, about 20 percent of signs that received glances could not be recalled 2 s after they were passed. A field test that used an eye-tracking device and asked drivers to recall selected signs did not show a predictable effect of sign environment on glance probability or recall. However, two other psychophysical testing methods showed that background environment does influence sign detectability, especially regulatory sign detectability. With light-colored or cluttered surrounds, speed limit signs were about half as detectable as they were with dark, uncluttered surrounds. With the surrounds explored in these studies, warning sign detectability did not vary by predictable or practically significant amounts as a function of background. However, this should not be taken to mean that warning sign detectability would not be markedly degraded by more complex surrounds than those explored in these studies. In particular, the present results should not be used as support for posting more than one sign on a roadside support, as the literature suggests that crowding would reduce readability in that situation.

Conclusions and Recommendations

Before drivers can behaviorally respond to signs, they must detect them. The sign detection angle appears to be a good measure of detectability. In these studies, the sign background affected detectability angle, but it was not clear whether the effects of background were the result of the general background or the part of the background that was immediately around the signs (e.g., within 2° of visual angle). Studies using the method of limits and the laboratory techniques used here should be useful in clarifying this issue. These studies should also assess the effectiveness of various conspicuity enhancements in mitigating the degrading effects of cluttered background scenes. Mitigation strategies that should be evaluated include increasing sign size and adding yellow plaques to regulatory signs.

This research suggests that in low-workload situations drivers can read speed limit signs that are 9° away from their gaze direction. Figure 32 shows that at 85 ft (26 m) a speed limit sign with 12-inch (30 cm) numerals that is located 12 ft (3.7 m) from the edge of the traveled way can be read without a glance away from the forward roadway. At distances closer to the sign than 85 ft (26 m), a glance away from the forward roadway would be necessary. At 255 ft (77.8 m), the sign would fall within 3°, or forward gaze, and would be legible while gazing at the forward roadway.

A vehicle is depicted in plan view traveling in the right lane of a two-lane road. Speed limit signs are depicted on the side of the road 12 ft from the road edge and at various distances from the vehicle. The sign 85 ft (26 m) ahead is 9 degrees to the right of the driver's forward view. The sign 170 ft (52 m) ahead falls within 6 degrees of the driver's forward view. The sign 255 ft (77.8 m) ahead falls within 3 degrees of the driver's forward view.
Figure 32 . Illustration. Relationship of viewing distance and visual angle offset from the driver's forward view of the roadway.

Legibility is a function of several factors. Within 3° of forward vision, high-contrast letters should be legible to most drivers with the MUTCD-recommended 1 inch (3 cm) of character height for every 30 ft (9 m) of viewing distance. Thus, a warning sign with 6-inch (15-cm) letters should become legible at 180 ft (55 m) and may be read without a glance directly to the sign at that distance. However, at a distance of 85 ft, a warning sign with 3-inch (8-cm) letters would require a glance away from the forward roadway to place the sign within foveal vision.

The MUTCD suggests that TCDs should be in the road user's view and that location and legibility should provide adequate time for response.(36) The results of this study suggest that the field of view for sign detection exceeds 60° under favorable conditions (i.e., low-clutter background that contrasts with sign color). Attention should be given to ensuring that signs stand out from their background. The study used the speed limit sign as an exemplar of regulatory signs. Intersections are a common location for black-on-white regulatory signs such as lane and turn restrictions. The need to place these signs on mast arms or posts in the immediate intersection environment often dictates the use of smaller signs and limits the ability to control the background and proximity to other signs. In these cases, strong consideration should be given to increasing the conspicuity of safety-critical signs (e.g., no U-turn and no turn on red). Many intersections present drivers with challenging visual environments comparable to the parking lot background used in this study's sign detection experiment. Unlike the observers in the sign detection experiment, drivers have multiple visual tasks to perform and will often lack the spare capacity to detect small signs or read those signs in their peripheral vision. To ensure that drivers look at and read safety-critical signs, every effort should be made to make the signs as large as possible and, if necessary, to add conspicuity enhancements such as the yellow notice plaque (W16-18P).

MUTCD section 2A.04 cautions against the excessive use of signs.(36) Warning sign prevalence may detract from warning sign effectiveness. To be effective, warning signs should specify a specific appropriate response that drivers can identify. The glance and recall findings in this study may be the result of overuse of occasional hazard signs for which no appropriate response is apparent and for which the actual hazard is rarely present (e.g., blind pedestrian, disabled pedestrian). One warning sign on the data collection route (not used as a target) was a curve warning sign with a speed advisory placard for 35 mi/h. Less than 85 ft (26 m) downstream from this sign and before the curve hazard was a 45-mi/h speed limit sign, an increase from the preceding 35-mi/h limit. At a minimum, increases in speed limits should not be placed upstream of curves where the speed advisory is less than the upwardly revised speed limit. Further down that same road, on a long tangent, was the slippery when wet warning sign that was comprehended by only 25 percent of the drivers who could recall it. That sign could benefit from a placard to explain its meaning and the expected driver response, such as an advisory speed for wet road conditions. It was not clear whether the area beyond the sign was extraordinarily slippery when wet or whether the sign was intended as a general reminder. In any case, signs such as these are likely to result in less than desirable attention to other warnings.

Regulatory signs of a general nature (e.g., littering prohibited) should be used very conservatively. One large sign at a town entrance is likely to be more effective than many small signs placed at intersections.

MUTCD section 2A.06 allows the use of word messages not included in the manual and does not require experimentation for word messages.(36) The present research suggests that the appropriate use of such warnings is effective. The parked vehicle ahead warning was particularly effective. That sign was on a two-lane road, upstream of where the road widened slightly and on-street parking was allowed. All five of the drivers who rated themselves unfamiliar with that stretch of road scored a glance to that sign and recalled it. Of the 11 drivers who rated themselves as familiar with the stretch of road, only 4 did not correctly recall the sign, although 3 of those 4 scored a look to it. This contrasts, for instance, with the deer warning symbol sign that was looked at by only 4 of 26 participants and recalled by only 4 of 16 who were asked to identify it. Immediate hazard signs, even text signs, appear to capture attention better than occasional warning signs. This does not imply that occasional warning signs should not be used, but rather, that they be used judiciously.

MUTCD table 2C-4 provides suggested distances upstream of a hazard where the warning sign for the hazard should be placed.(36) Placement further upstream than suggested is strongly discouraged because drivers' memories are short. Where the subject of the warning (e.g., intersection, crosswalk) is not visible 2 s upstream of the warning sign, a distance placard is strongly recommended. As noted previously, warnings for which no response is apparent are quickly forgotten.

Additional research is needed to characterize the influence of environment in TCD detection and awareness. MDS may be a useful tool in that effort. However, other methods of characterizing the TCD environment are also needed.


Thanks to Pascal Beuse, Dana Duke, and Jason Kennedy for their efforts in conducting the eye glance and recall field study. Thanks to Mike Baumgartner, Ryan Cartwright, Peter Chou, and Jason Williams for their efforts in enabling the laboratory detection and identification studies. Thanks again to Dana Duke for his creative contributions to the outdoor data collection effort.


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