U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590

Skip to content
Facebook iconYouTube iconTwitter iconFlickr iconLinkedInInstagram

Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information
Back to Publication List        
Publication Number:  FHWA-HRT-16-037    Date:  June 2016
Publication Number: FHWA-HRT-16-037
Date: June 2016


Active Traffic Management: Comprehension, Legibility, Distance, and Motorist Behavior in Response to Selected Variable Speed Limit and Lane Control Signing

Chapter 3. Sign Field testing

A subset of the ATM signs used in the laboratory study were selected for further field testing. This test was for a large number of CMS type signs of which the ATM signs were a small subset. The principal focus of the field test was to obtain legibility distances for the signs. The data were collected on a closed course while participants drove an instrumented vehicle that was equipped with a dashboard-mounted eye-tracking system.


Test Facility

Testing was conducted on a 30-ft- (9-m-) wide drag strip. A CMS was placed on the left side of the drag strip 1,250 ft (376 m) from the start line. Traffic cones were placed on the track to form a 12-ft- (3.6-m-) wide lane that curved first from the left to the right side of the track, then back to the left, and ended in the middle of the track at the CMS location. The arrangement of the traffic cones is shown in figure 40. The purpose of the curved path was to require participants to attend to lane keeping in addition to messages displayed on the sign.

Figure 40. Photo. Layout of course on drag strip. This figure shows a two-lane drag strip with cones dividing the lanes.

Figure 40. Photo. Layout of course on drag strip.

Changeable Message Sign

The CMS was a Daktronics® VF-2320 full color (RGB) matrix display with a 0.79-inch (20‑mm) pixel pitch. The display surface was 4 by 4 ft (1.2 by 1.2 m), which accommodated a 64- by 64-pixel display. Figure 41 shows the sign as deployed for testing on the drag strip.

Figure 41. Photo. The CMS used in the study. This figure shows an example of the changeable message sign (CMS) used in the study. The CMS is mounted on a trailer on the left side of the road. There are three cones the roadway.

Figure 41. Photo. The CMS used in the study.

The brightness of the sign was set at 100 percent because the tests were conducted in daylight between 9 a.m. and 1 p.m. in fall 2012. At the 100-percent brightness setting, a white stimulus 12inches (30 cm) in diameter measured between 3,065 and 3,502 fl (10,500 and 12,000cd/m2), depending on which location on the display was measured. A red stimulus measured a mean of 1,051 fl (3,600 cd/m2) and an amber stimulus a mean of 3,094 fl (10,600cd/m2). Laboratory testing, reported elsewhere, showed that the display was compliant with National Electrical Manufacturers Association standards for LED color displays intended for highway applications.

Five of the signs evaluated in the laboratory test as described in chapter 2 were selected for testing in the field. Table 12 presents a list of the signs that were tested.

Table 12. List of ATM signs used in the field test.

Sign Source
40 MPH
45 MPH
40 MPH
Red X Lane Closed
Yellow X Lane Closing 1 mi

MPH = Miles per hour.
WSDOT = Washington State Department of Transportation.
MNDOT = Minnesota Department of Transportation.

Research Vehicle

Participants drove a 2007 Jeep® Grand Cherokee that was equipped with a dashboard-mounted three-camera eye-tracking system. The system sampled eye vectors and head position at 60 Hz. Three scene cameras mounted on the roof of the Jeep®, directly over the driver's head, recorded about 80 degrees (horizontal) of the driver's view of the road ahead. In post processing, the scene camera view was merged with the eye-tracking vectors. In addition to the eye-tracking data, direction-measuring equipment, Global Positioning System device, and accelerometer data were recorded and synchronized.(11)

The sound recording capability of the eye-tracking system was not functional at the time these tests were conducted. Therefore, to capture participants' verbal responses, the experimenter operated a handheld voice recorder during each trial. Recordings were started as the participant maneuvered the research vehicle to the start line and stopped after the participant correctly spoke the message on the sign.


Participants were instructed to begin each trial at the approach to the left start line. There they waited for an experimenter in the back seat to signal that the data recording equipment was ready. At that point, participants were to briskly accelerate to 25 mi/h (40 km/h) and maintain that speed while they attempted to read the CMS message. Participants were instructed to read the message aloud as soon as practicable while still maintaining 25 mi/h (40 km/h) and staying within the marked lane.

Two measures were extracted from the voice recordings of participants: (1) trial duration—time from the beginning of a trial until the participant completed correctly reading aloud the message on the sign and (2) response duration—the time between when the participant began repeating the message aloud and when the participant completed reading the message. All measures were to the nearest second. Response durations of zero could result when responses were rounded to the nearest second. Trials always began with the vehicle at a full stop. The beginning of each trial was marked when the sound of the engine revving was noted in the recordings.


Useable data were obtained from nine participants (six males and three females). The mean age of participants was 39 years old (range of 19 to 61 years old). Participants were paid $40/h for participating between 3 and 5 h. All drivers were licensed in the State of Virginia.


Table 13 presents mean legibility distances for the five ATM signs. The mean legibility distances for the speed limit signs were close to the maximum legibility distance in this test (1,250 ft (381m)). The yellow X had the lowest mean legibility distance and the highest STD. The greater legibility distance and STD in responding to this sign were in part due to the method used to obtain a response. As previously discussed, a response did not conclude until the participant completed reading the message. Thus, if a participant struggled in interpreting the true meaning of the yellow X sign (i.e., he or she would continue to drive closer to the sign location), then the response duration would be longer. In turn, the legibility distance would decrease.

Table 13. Legibility distance measures for selected ATM signs.

Sign Mean (ft) STD (ft) Minimum (ft) Maximum (ft)
Regulatory SPEED_WSDOT
1,220.67 16.40 1,213.33 1,250.00
1,225.56 18.93 1,176.67 1,250.00
1,243.89 14.97 1,213.33 1,250.00
Yellow X MUTCD
1,040.48 305.10 480.00 1,250.00
1,187.14 103.09 956.67 1,250.00

1 ft = 0.305 m.


Highway guide signs use a letter height of 16 inches (40.64 cm). This letter height is based on a legibility distance of 30 ft/inch (9.14 m/2.54 cm) of letter height. This is a conservative guideline to accommodate a driver with 20/40 vision. A factor in the design of signs and the letter height used is the available time to read a sign at highway speeds. The yellow X had an average legibility distance of 1,040 ft (316.99 m). At a speed of 65 mi/h (105 km/h), the reading time is approximately 11 s, which is greater than the design guideline minimum of 8 s. The ATM signs using the new CMS displays with high resolution result in legibility distances that exceed design guidelines for non-LED signs (e.g., highway guide signs).



Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000
Turner-Fairbank Highway Research Center | 6300 Georgetown Pike | McLean, VA | 22101