Speed Feedback and Changeable Message Signs

Image Source: Iowa DOT

DEFINITION AND DESCRIPTION

Dynamic speed feedback signs contain devices that measure the speeds of approaching vehicles, display vehicle speeds.  Changeable message signs (CMSs) can include messages (e.g., Slow Down) that instruct drivers to modify their speeds if they exceed a predetermined threshold. Sign installations can be permanent or temporary. They are often used in transitions zones that link high-speed to low-speed contexts or on approaches to curves on high-speed roadways to help prevent roadway departure crashes. Installations on two-lane rural horizontal curves are associated with CMFs of 0.93 – 0.95 (Hallmark et al., 2015).

Implementation Categories

CONSIDERATIONS

• Signs can be mounted permanently on a standard metal pole.
• Must meet applicable MUTCD requirements. See Section 2C.13 for Speed Feedback Signs and Chapter 2L for CMSs.
• When used to display the speed of an approaching vehicle in relation to the posted speed limit, the Vehicle Speed Feedback (W13-20aP) plaque shall be mounted below a Speed Limit sign.
• When used to supplement a horizontal alignment warning sign advisory speed, the Vehicle Speed Feedback sign (W13-20) shall be an independent installation near the point of curvature of a horizontal curve.
• CMSs can display a warning or a simple message (e.g., TOO FAST or XX mi/h).
• Permanent installations should have self-contained power (e.g., solar).
• If displaying the vehicle speed, consider an upper limit threshold above which the sign no longer displays speed, so drivers do not “test” the sign.

APPLICATION

• Mobile signs may be used for construction and maintenance work zones.
• Mobile or permanent signs may be placed in speed transition zones along approaches to communities.
• Evaluate using permanent installations at locations with a higher probability of roadway departures (e.g., long tangents before curves with significantly lower design speeds, locations with a history of roadway departures).
• Determine the speed threshold that will activate the sign (e.g., 5 mph over the speed limit or the curve advisory speed).
• Place signs based on perception-reaction time, deceleration, and expected reduction in speed so the driver has time to react and adjust their speed. Provide a clear line of sight to signs.

example

Image Source: MUTCD, 11^th Edition (left), CTRE at Iowa State University (right)

Complimentary SSA Strategies

• Curve Widening
• Enhanced Signing