When a lane is closed on a two-lane, two-way roadway, flaggers are typically used to control the flow of traffic through the work zone.
While various measures have been implemented in recent years to improve the safety and effectiveness of flaggers, crashes involving flaggers still occur and quite often result in serious injury.
The 2009 Manual on Uniform Traffic Control Devices (MUTCD) includes automated flagger assistance devices (AFADs), which are designed to be remotely operated by a flagger positioned outside of the travel lane. There are two types of AFADs: Stop/Slow and red/yellow lens. Stop/Slow AFADs alternately display a Stop sign to stop traffic and a Slow sign when traffic may proceed. A Wait on Stop sign must be displayed with a Stop/Slow AFAD. A Go on Slow sign and gate arm are optional.
Red/yellow lens AFADs alternately display a steadily illuminated circular red lens and a flashing circular yellow lens to control traffic. Unlike the Stop/Slow AFAD, the red/yellow AFAD must include a gate arm. A Stop Here on Red sign also is required. To stop traffic, the red/yellow lens AFAD displays the steadily illuminated circular red lens and the gate arm is in the down position. When traffic may proceed, the AFAD displays a flashing circular yellow lens, and the gate arm is in the upright position. A steady circular yellow indication must be provided to transition between the display of the flashing circular yellow indication and the display of the steady circular red indication.
While AFADs may increase the safety of flaggers, there were concerns that motorists may misunderstand AFADs and proceed before it is safe to do so. As part of a recent Texas Department of Transportation (TxDOT) project, Texas A&M Transportation Institute (TTI) researchers conducted motorist surveys at Department of Public Safety (DPS) offices to assess motorist understanding of AFADs. In addition, researchers conducted field studies at lane closures on two-lane, two-way roadways to evaluate the operational and safety effectiveness of AFADs relative to the use of flaggers.
Seeing the signs
Based on anecdotal evidence from previous applications, researchers believed that the current supplemental signs used with Stop/Slow AFADs (i.e., Wait on Stop [required] and Go on Slow [optional]) may not be understood by motorists. So researchers evaluated motorist comprehension of the current supplemental signs and several alternative supplemental signs developed by researchers. Overall, researchers surveyed 476 motorists. While a participant viewed a graphic of a Stop/Slow AFAD in the stop condition without a gate arm on a laptop computer, researchers asked a series of questions to determine motorist understanding of the situation.
Of particular interest were the participants’ answers to the question “Would you stop and go (incorrect) or would you remain stopped until otherwise indicated (correct)?” For the Wait on Stop sign, almost one-quarter of the participants stated they would stop and proceed like at a standard stop sign. Similarly, for the Wait on Stop/Go on Slow sign, approximately one-third of the participants stated they would stop and proceed. In contrast, less than 10% of the participants stated they would stop and then proceed for the Wait on (stop symbol)/Go on (slow symbol) sign.
Researchers were concerned that the signal indications used with the red/yellow lens AFAD may cause confusion since they differ from traditional signal displays (i.e., red, yellow and green for a traffic signal or red and green for freeway-entrance control signals). While a participant viewed a video sequence of a red/yellow lens AFAD on a laptop computer, researchers asked a series of questions to determine comprehension.
The findings showed that participants understood that the steady red signal with the gate arm down meant stop, and the flashing yellow signal with the gate arm up meant to proceed. However, most participants did not understand the difference between the flashing and steady yellow signals. Even so, the use of the gate arm appeared to inform motorists about when to proceed and when to stop.
The proper treatments
In the summers of 2010 and 2011, researchers conducted field studies at 20 lane closures on two-lane, two-way roadways in Texas. Most of the sites were in rural locations, but some sites were in urban areas. The work activities consisted of ditch maintenance, chip seal, grading, patching, base repair and bridge work. The 2009 average annual daily traffic (AADT) ranged from 220 to 5,100 vehicles per day (vpd), with the majority of the sites (55%) having less than 2,000 vpd. The length of the lane closures ranged from approximately 300 ft to 1 mile, with three-quarters of the sites less than 2,500 ft in length. The speed limit ranged from 35 to 70 mph.
Researchers evaluated the following treatments during the first summer:
A flagger with a Stop/Slow paddle at both ends of the lane closure;
A Stop/Slow AFAD with a Wait on Stop sign at both ends of the lane closure; and
A red/yellow lens AFAD with a Stop Here on Red sign and a gate arm at both ends of the lane closure.
The AFAD treatments initially studied represented the minimum requirements in the MUTCD.
After reviewing the Stop/Slow AFAD motorist comprehension survey results and the initial field study data, researchers decided to evaluate the following treatments during the second summer to assess the impact of supplemental signs and a gate arm with the Stop/Slow AFAD:
A Stop/Slow AFAD with a Wait on Stop sign and a gate arm at both ends of the lane closure;
A Stop/Slow AFAD with Wait on Stop and Go on Slow signs at both ends of the lane closure;
A Stop/Slow AFAD with Wait on Stop and Go on Slow signs and a gate arm at both ends of the lane closure;
A Stop/Slow AFAD with Wait on (stop symbol)/Go on (slow symbol) signs at both ends of the lane closure; and
A Stop/Slow AFAD with Wait on (stop symbol)/Go on (slow symbol) signs and a gate arm at both ends of the lane closure.
Since the last two treatments included signing not approved in the MUTCD, TxDOT submitted a request to Federal Highway Administration (FHWA) to experiment with the alternative supplemental signing. FHWA approved this request on June 1, 2011.
All treatments with a gate arm included a flag on the end of the gate arm, and all treatments included cones on the centerline immediately upstream of the treatment (the exact number and spacing of cones varied). Researchers used the recommended advance-warning-sign sequence in the MUTCD with all treatments.
During the second summer noncompliance at the Stop/Slow AFAD treatments without a gate arm raised safety concerns that led researchers and TxDOT to terminate further field evaluations of AFADs without a gate arm. Overall, researchers collected observation data for 102 hours and 1,708 stop periods.
The primary measure of effectiveness for the field studies was compliance with the treatment’s instructions (flagger or AFAD). Researchers were typically located near the work area but were always out of view from motorists. In addition, the flagger controlling the AFADs was not in the immediate vicinity of the treatment. Instead, the flagger was usually located at the midpoint of the work area so he/she could see both directions of traffic.
Table 1 contains the violation rate for each treatment. The violation rate represents the number of violations per 100 stop cycles. All of the motorists stopped by a flagger complied with the flagger’s instructions; thus, the violation rate was zero. The violation rate for the Stop/Slow AFAD with a Wait on Stop sign and no gate arm was the highest (6.7), and was significantly different from the violation rate for the red/yellow lens AFAD (2.2). Adding a gate arm to the Stop/Slow AFAD decreased the violation rate to 4, which was not significantly different from the red/yellow lens AFAD. Once a gate arm was added to the Stop/Slow AFAD, all of the treatments resulted in similar violation rates independent of the supplemental signs used (3.2, 3.8 and 4).
Researchers did not specifically design the study to examine the impacts of traffic volume and work-zone length. However, researchers did examine the general trends associated with these variables and found weak correlations between violations and traffic volume and violations and work-zone length. Based on these data, the violation rate at AFADs decreased as the traffic volume increased. Discussions with TxDOT personnel revealed that motorists on low-volume roads are typically locals who know the small likelihood of encountering an oncoming vehicle and thus may choose to disregard the AFAD’s instructions, especially when there is no apparent oncoming traffic. Researchers also found that as the work-zone length increased so did the violation rate. This trend may be attributed to the longer wait times motorists incur as the work-zone length is increased. Again, while researchers can identify general trends from the relationships, the findings were highly variable.
Researchers and TxDOT personnel also noted that without a gate arm there were more deliberate violations of the Stop/Slow AFAD, especially when the queue of vehicles going in the same direction was visible to approaching motorists. On several occasions, a motorist would approach the Stop sign displayed on the AFAD, hesitate and then decide to proceed to catch up with the back of the queue. This behavior did not occur when the gate arm was used, since the gate arm and channelizing devices on the centerline blocked the motorist’s path.
Both are fine
To assess the operational and safety effectiveness of AFADs relative to the use of flaggers, researchers conducted motorist surveys at DPS offices and field studies at lane closures on two-lane, two-way roadways in Texas. The motorist survey findings showed that the experimental Wait on (stop symbol)/Go on (slow symbol) supplemental sign resulted in the highest percentage of participants who understood to stop and remain stopped until the Stop/Slow AFAD indicated that it was safe to proceed. Likewise, this experimental sign yielded the lowest percentage of participants who stated they would stop and then proceed like at a standard Stop sign. However, for all of the Stop/Slow AFAD treatments evaluated, a portion of the participants indicated they would have stopped and then proceeded instead of waiting until the AFAD displayed the Slow sign.
Even though there is evidence of a lack of understanding of the difference between the flashing and steady-burn yellow signals of the red/yellow lens AFAD, the use of the gate arm informs motorists when to proceed and when to stop (raised when the signal changes to the flashing yellow signal and lowered when the signal changes to steady red). In addition, more participants understood the difference between the two yellow signals when they observed the AFAD cycle in the following order: flashing yellow, steady-burn yellow and red. This is important because motorists approaching the AFAD during the proceed condition (flashing yellow) need advance warning that the AFAD is about to change to the stop condition (red).
The field study findings showed that the violation rate for the Stop/Slow AFAD with a Wait on Stop sign without a gate arm (minimum requirements in the MUTCD) was the highest and was significantly higher than the violation rate for the red/yellow lens AFAD (which requires a gate arm). Adding a gate arm to this Stop/Slow AFAD decreased the violation rate such that it was not significantly different from the red/yellow lens AFAD. Once a gate arm was added to the Stop/Slow AFAD, the supplemental signs evaluated did not appear to impact compliance.
Overall, the research findings show that some motorists will violate AFADs, especially when the queue of vehicles going in the same direction is visible to the stopped motorist. However, for all the documented violations the flagger was able to stop the motorist before they encountered oncoming traffic. Therefore, researchers believe that both types of AFAD (Stop/Slow and red/yellow lens) may be used to control traffic at lane closures on two-lane, two-way roadways. Agencies implementing AFADs need to be aware that violations will occur, especially when AFADs are initially deployed. Thus, agencies should use public-outreach techniques such as news stories, the Internet and social media in areas where it will deploy AFADs to notify and educate motorists.
Researchers did not make any specific recommendations regarding the red/yellow lens AFADs, since the research findings supported the current language in the MUTCD. For Stop/Slow AFADs, researchers made the following recommendations:
A gate arm should be required when using Stop/Slow AFADs to increase compliance; and
The Wait on (stop symbol)/Go on (slow symbol) supplemental sign should be used with Stop/Slow AFADs, instead of the current recommended signing, to improve motorist understanding of Stop/Slow AFADs. ST
