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OCR for page 47
HFG CHANGEABLE MESSAGE SIGNS Version 1.0
CHANGEABLE MESSAGE SIGNS FOR SPEED REDUCTION
Introduction
CMSs for speed reduction refers to situations in which a reduction in the speed of the traffic flow is desirable due to
potential hazards, work zones, adverse weather conditions, incident control, or heavy congestion. Applications that
are temporary or variable in nature are the primary candidates for using a speed-reduction CMS. Areas that
experience recurring heavy peak traffic also can benefit from the proper application of a speed-controlling CMS.
Design Guidelines
General CMS Applications for Speed Reduction:
Provide a reason for the reduced speed.
Limit the use of safety campaign messages.
Use CMS with radar for speed reduction:
"You Are Speeding/Slow Down" is an effective message for speeders (5).
"Your Speed/XX mph" is the MUTCD-approved text for displaying approach speeds.
Work Zone CMS Applications for Speed Reduction:
In work zones over 3500 ft, use a second CMS partway through.
For extended work (i.e., 1 year), use CMS for the project opening days and after major condition changes.
Use passive controls at other times.
Place the first CMS 500-1000 ft upstream from the hazardous location within the work zone after the first
advance sign.
Place signs away from other work zone signs, ramps, intersections, or lane-closure tapers.
Based Primarily on Based Equally on Expert Judgment Based Primarily on
Expert Judgment and Empirical Data Empirical Data
SIGN PLACEMENT IN A WORK ZONE
Source: adapted from the MUTCD (6)
MAXIMUM SPEED REDUCTIONS IN WORK ZONES
Roadway Type Speed Reduction (mi/h)
Rural two-lane, two-way highway 10-15
Rural freeway 5-15
Urban freeway 5-10
Urban arterial 10-15
Source: Richards and Dudek (4)
19-12
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HFG CHANGEABLE MESSAGE SIGNS Version 1.0
Discussion
General applications: Speed-reduction CMSs are used to reduce speeds during a wide range of events such as
potential hazards, adverse weather conditions, traffic incidents, and heavy congestion. When CMSs are provided to
reduce driver speeds, compliance is increased when a reason for the reduced speed is displayed (1). These signs are
still effective after 7 weeks of exposure, continuing to cause significant speed reductions (2). In a simulator study
by Jamson and Merat (3), little change in driver behavior was observed when the safety campaign message "Watch
Your Speed" was displayed (maximum 0.5 mi/h speed reduction). However, it was found using eye-tracking data
that drivers continued to look at CMSs along the route even though the message was repeated. Drivers who
witnessed 33% of the CMSs on the route displaying safety campaign messages changed lanes significantly faster in
response to an incident message than those who saw either all blank CMSs or 100% of the CMSs showing safety
messages. These drivers also spent significantly more time looking at the incident message than any other group.
An FHWA Policy Memorandum states that driver safety campaign messages should be limited to a few weeks so
that drivers do not begin to ignore them (see "When to Use Changeable Message Signs" on page 19-2).
CMSs can also use radar to reduce speeds. Garber and Srinivasan (2) refer to a number of studies that show CMS
with radar to be effective in reducing passing vehicle speeds. The message "You Are Speeding/Slow Down" proved
to be the most effective message for reducing speeds (5). This message reduced average speeds, 85th percentile
speeds, and traffic speed variance by statistically significant amounts. The MUTCD states that for these signs, the
legend "Your Speed/XX mph" or something similar should be shown (6).
Work zone applications: CMSs have a limited range of effectiveness. The first CMS should be positioned 500 to
1000 ft upstream from the hazard in a work zone to give drivers time to react before reaching that hazard. However,
this distance cannot be too long because drivers need to remember the message and maintain the reduced speed
when they reach the hazard. In longer work zones, drivers tend to increase their speeds when they near the end of
the zone, far away from the first CMS (2). Thus, if hazards continue to exist throughout a long zone, a second CMS
may be needed.
The visibility and prominence of CMSs are important. Ideally, drivers will not be overloaded with information and
will have sufficient available attention to focus on the CMS (4). Thus, the guidance is to place the CMS away from
work zone signs, and out of high driver workload areas such as ramps, intersections, or lane-closure tapers.
Credibility is a general issue with CMSs that also applies to the application of CMSs in work zones. The selection
of an unreasonably low speed causes drivers to lose respect for the signs, which leads to a loss of credibility (4).
This loss of credibility can lead to reduced effectiveness of signs at other sites as well.
Richards and Dudek (4) report that drivers will only slow down a limited amount regardless of the posted limit. The
reductions in average work zone speeds were found to be 5-20 mi/h, depending on the site. Thus, Richards and
Dudek suggest maximum speed reductions in work zones as shown in the table on page 19-12.
Design Issues
Speed reductions as supported by CMSs can cause reductions in roadway capacity and congestion (4).
Cross References
Changeable Message Signs, 13-6
When to Use Changeable Message Signs, 19-2
Displaying Messages with Dynamic Characteristics, 19-10
Key References
1. Pedic, F., and Ezrakhovich, A. (1999). A literature review: The content characteristics of effective VMS. Road & Transport Research, 8(2),
3-11.
2. Garber, N.J., and Srinivasan, S. (1998). Effectiveness of Changeable Message Signs in Controlling Vehicle Speeds in Work Zones--
Phase II. (FHWA/VTRC 98-R10). Charlottesville: Virginia Transportation Research Council.
3. Jamson, A.H., and Merat, N. (2007). The effectiveness of safety campaign VMS messages--A driving simulator investigation. Driving
Assessment 2007: 4th International Driving Symposium on Human Factors in Driver Assessment, Training, and Vehicle Design, 459-465.
4. Richards, S.H., and Dudek, C.L. (1986). Implementation of work-zone speed control measures. Transportation Research Record, 1086,
36-42.
5. Garber, N.J., and Fontaine, M.D. (1996). Controlling Vehicle Speeds in Work Zones: Effectiveness of Changeable Message Signs with
Radar. (UVA/529242/CE96/102). Charlottesville: University of Virginia.
6. FHWA (2009). Manual on Uniform Traffic Control Devices for Streets and Highways. Washington, DC.
19-13
