ITE Journal - May 2020 - 41

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peed limit violations are a major public concern, especially along streets near schools
and parks where substantial pedestrian/bicycle-to-vehicle conflicts are experienced.
To compensate for limited staff and budget resources, public agencies have turned
to radar speed feedback signs that measure driver speed compliance with posted

speed limits. Roadside equipment like radar speed signs are used to supplement or substitute for
enforcement staff.
The City of Campbell, CA, USA is comprised of 43,000 residents
and spans six square miles. The city uses radar speed feedback signs
to manage traffic speeds. The "conventional wisdom" is that radar
signs are effective when first installed, but over time the signs lose
their effectiveness. Therefore, the hypothesis of this study is that
radar signs do not maintain their effectiveness over time.

Literature Review
In a paper titled "Speed feedback signs as a tool to manage demand
for lower residential speeds," lead author Churchill states, "Results
indicated that average speed during...trailer deployment reduced
by 1.59 kilometers per hour [km/hr] [1 mile per hour (mph)] to 5.64
km/hr [3.5 mph] depending on the location, compared to before
installation period."1 This study adds, "The speed level was back to
the before installation level after four weeks of [sign] installation."
Churchill cites the paper "Long Term Effectiveness of Radar
Speed Display Boards Used in School Zones" authored by
Hildebrand et al, which evaluated the long term effectiveness of
speed display boards on speed reduction through school zones; one
week, two months, one year and four years after the installation.2
Before-after study results indicated that sustained and statistically
significant reduction in the average speeds ranging from 5-14 km/
hr (3.1-8.7 mph) were achieved, dependent mostly on the degree of
excessive speeding prior to installation. Mean speeds were reduced
consistently to 36-37 km/hr (22.4-23.0 mph) on a 30 km/hr (18.6
mph) zone. It was concluded that speed display boards have a
statistically significant long-term effect on reducing motorist speeds
through school zones."
In a paper entitled "Effective Deployment of Radar Speed
Signs" lead author Veneziano cites the results of past research
including permanent radar sign sites in school zones and additional
locations.3 For school zones, Veneziano noted six studies with mean
speed changes ranging between 1 and 7 mph (1.6 and 11.3 km/hr)
apparently within 12 months of sign deployment. For residential,
commercial, speed transition zones, Veneziano mentions two
studies in speed transition zones where mean speed changes

included a 6-8 mph (9.7-12.9 km/hr) reduction in one study, and
1-3.4 mph (1.6-5.5 km/hr) reduction in another, after 12 months of
deployment. It was unclear whether any of the past research went
beyond 12 months after sign deployment.

Test Set-up
This study is a longitudinal analysis of posted speed limit
compliance over a five-year period. The speeds of mixed traffic
were measured for various corridors in Campbell, CA. Note: street
segments in this study were not targeted for speed enforcement,
and should be treated as a stochastic influence on observed speeds.
Radar speed signs (30 inches [in.] x 42 in.) with 26.5 in. x 20
in. displays were installed per manufacturer specifications at 10
different locations. The radar speed signs varied in number of
lanes, street classification, adjacent land uses, and speed limits,
and were mounted away from stop signs or traffic signals to
capture the attention of motorists at their top speeds. The signs
fall under the U.S. Federal Communications Commission (FCC)
Part 15 rules and are permanently accurate to within one mph as
factory-calibrated.
Approach speeds of almost all vehicles are recorded on each
subject street. For vehicles traveling close to each other or hidden
behind another vehicle the signs record only one vehicle and
speed. The signs record the last speed of a vehicle before they begin
displaying the speed of another vehicle. Campbell streets surveyed
in this study are flat and straight; the equipment detects vehicle
speeds up to a range of 700 ft. (213 meters [m]). When a sign is
set up and positioned to aim the radar gun inside the sign in the
direction specified by the manufacturer's instructions, a sign will
detect and display the two closest lanes at the same distance; a third
lane over may start displaying at a closer distance. The closest lane
will stop recording and displaying speed approximately 50 feet (ft.)
(15 m) earlier.
Table 1 summarizes the setting of each surveyed street. Study
streets were selected to represent a range of street classifications and
number of lanes. Figure 1 shows the sign locations.
w w w .i t e.or g

May 2020

41


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