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Selecting an Appropriate Treatment 21
(Credit: FHWA, 2006)
(a) Reduced lane width, added rumble strips, and added pavement markings on major road
(Credit: FHWA, 2006)
(b) Installed mountable splitter island with stop signs on minor road approaches
Exhibit 3-4. FHWA low-cost treatment concepts.
several years of data are needed to determine the potential effects the treatments had on road-
way safety. (FHWA, 2006)
3.5 Treatment Considerations
This section provides information to aid in screening treatments and to identify one or more
treatments that may be most appropriate for a particular condition. Exhibits 3-5 and 3-6 pro-
vide a summary of the considerations for each treatment. Treatments for which specific testing
has been conducted are shown in the shaded sections.
As described in the following sections, Exhibit 3-5 summarizes typical uses of the treatments,
indicates how they may be applied at high-speed intersections, indicates whether research was
found to document their effectiveness, provides order-of-magnitude cost relationships, and
indicates the treatments' experimental status with respect to the MUTCD. Exhibit 3-6 provides
the documented treatment applications, their effectiveness, and noteworthy considerations.
Section 4 provides supplemental information, diagrams, and photos for each treatment. The
information provided in Section 4 represents the best available data based on the extensive lit-
erature review and field testing conducted under NCHRP Project 3-74.
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22 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections
Treatment Documented Uses Potential High-Speed Intersection
Applications
Dynamic Warning Horizontal curves Rural, unsignalized intersections
Signs Work zones Use caution in high driver-workload contexts
Transverse Pavement Traffic circle and stop-controlled intersection History of high-speed crashes
Markings approaches
Horizontal curves, bridges, freeway off-ramps
Work zones
Transverse Rumble Approaches to intersections Use caution in noise-sensitive contexts and with
Strips Approaches to toll plazas motorcycle traffic
Longitudinal Rumble Rural highways Consider impacts to bicycle traffic
Strips
Wider Longitudinal Crash history involving curves, hills, roadway cross Run-off-road crashes
Pavement Markings section Use caution as increased visibility may lead to
Work zones increased speeds
Roundabouts Rural highways Rural Highways
Transition areas Gateways
Use caution with steep grades, unusual geometry,
constrained right-of-way
Approach Curvature Roundabout approaches Stop- and yield-controlled approaches
Use caution with run-off-road crash history,
grades
Splitter Islands Roundabout and stop-controlled approaches Minor approaches of T-intersections and two-way,
stop-controlled intersections
Speed Tables/ Plateau Local and collector streets Stop-controlled approaches
Plateaus installed on facilities with 35 to 50
mph speeds in New Zealand
Reduced Lane Width Work zones Uncontrolled approaches
Low-speed urban and residential locations
Rural two-lane highways
Visible Shoulder Rural two-lane highways Wide shoulders
Treatments Run-off-road crashes
Roadside Design Transition areas Transition areas
Features (Including Adjacent to roadway or intersection beyond clear Consider sight distance at driveways
Gateways and zone and line of sight
Landscaping)
Note:
Shaded rows indicate treatments tested through NCHRP Project 3-74.
Exhibit 3-5. Treatment application summaries matrix.
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Selecting an Appropriate Treatment 23
Treatment Function Documented Documented Cost Experimental
Speed Safety Status
Reduction Improvement
Dynamic Warning Encourage deceleration Yes Yes $$ No
Signs Reduce comfortable approach speed
Transverse Pavement Improve intersection visibility Yes Yes $ Yes
Markings Alert drivers to upcoming intersection
Transverse Rumble Encourage deceleration Yes No $$ No
Strips Reduce comfortable approach speed
Longitudinal Rumble Reduce comfortable approach speed No data Yes* $$ No
Strips
Wider Longitudinal Increase visibility and demark the No data No data $ No
Pavement Markings intersection influence area
Roundabouts Reduce speed through intersection Yes Yes $$$ No
Reduce conflicts
Approach Curvature Reduce comfortable approach speed Yes* Yes $$$ Yes
Splitter Islands Improve intersection visibility No data Yes $$ Yes
Alert drivers to upcoming intersection
Speed Tables/Plateau Encourage deceleration Yes* Yes* $$ No
Reduce comfortable approach speed
Reduced Lane Width Reduce comfortable approach speed Yes* No $$ No
Visible Shoulder Alert drivers to upcoming intersection No data Yes $ No
Treatments Improve intersection visibility
Roadside Design Reinforce transitioning roadway Yes* Yes* Varies No
Features (Including environment
Gateways and Improve intersection visibility
Landscaping)
Reduce comfortable approach speed
Notes:
Shaded rows indicate treatments tested through NCHRP Project 3-74.
* Indicates data for low-speed intersections and/or roadway segments only.
Exhibit 3-5. (Continued).
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24 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections
Treatment Documented Applications Documented Speed Effects
Dynamic Warning High-speed 2 test sites (WA, TX) uncontrolled Mean speeds reduced up to 2.8 mph
Signs intersections approaches Initial reduction of 3 to 4 mph at two TX
sites, one site had sustainable results
Others Rural interstate work zones (SD, VA) 1.4 to 4 mph mean speed reduction in
Freeway ramps (MD, VA) work zones
School zones (TX) Reduction in the number of speeding
vehicles in work zones: 2025% for cars,
Curve approaches 40% for trucks
Transverse Pavement High-speed 5 test sites (OR) Up to 0.9 mph mean speed reduction
Markings intersections Stop-controlled and uncontrolled
approaches
Others Horizontal curves 20-30% speed reduction on segments
Roundabout approaches Peripheral lines almost as effective as full
Work zones transverse lines
Highway off-ramps
Bridges
Transverse Rumble High-speed 3 test sites (TX) uncontrolled approaches 1.3 mph mean speed reduction at the
Strips intersections perception-response time location
Others Some stop-controlled approaches More gradual deceleration
Toll plazas, horizontal curves, and work Increased speed variation
zones Not successful in significantly reducing
approach speeds
Longitudinal Rumble High-speed No data No data
Strips intersections
Others Shoulder or center of expressways, No data
parkways, and rural highways
Wider Longitudinal High-speed No data No data
Pavement Markings intersections
Others Work zones No data
Vertical and horizontal curves
Access-controlled highways
Toll roads
Roundabouts High-speed Many applications on high-speed and low- Geometry yields 13 to 17 mph 85 th -
intersections speed facilities percentile entry speeds
Others Roundabout type and design vary based
on facility speed, access, volumes, vehicle
composition, and other factors
Note:
Shaded rows indicate treatments tested through NCHRP Project 3-74.
Exhibit 3-6. Treatment effectiveness summaries.
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Selecting an Appropriate Treatment 25
Treatment Documented Safety Effects Key Considerations
Dynamic Warning No data Power supply
Signs Urban vs. rural applications
Reduction in rollover crashes on freeway ramps
Target speed
Coordinate with other treatments closer to
intersection
Transverse Pavement No data Driver familiarity
Markings Human factors considerations
50-65% crash reduction at roundabouts and high
crash locations
Transverse Rumble No data Noise impacts to adjacent land uses
Strips May make bicycling difficult
Marginal safety benefits at best
Vibrations can startle drivers and bicyclists, causing
quick and unsafe maneuvers
Maintenance
Longitudinal Rumble No data Truck traffic, if narrowing lane width
Strips Vibrations can startle drivers and bicyclists, causing
Reductions in injury, sideswipe, and opposite quick and unsafe maneuvers
direction crashes
Maintenance
46% reduction in shoulder encroachment
Wider Longitudinal No data Improved visibility, especially with older drivers
Pavement Markings
No data
Roundabouts All roundabout conversions: approximate 35% Operational performance
reduction in total crashes and 77% reduction in Right-of-way needs
injury crashes
Pedestrian and bicycle safety
Rural two-way stop conversions: approximate 72%
reduction in total crashes and 87% reduction in Gateway treatment
injury crashes
Note:
Shaded rows indicate treatments tested through NCHRP Project 3-74.
Exhibit 3-6. (Continued).
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26 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections
3.5.1 Typical Uses
Many of the treatments mentioned in Section 4 have not been documented as having been used
at high-speed intersections; however, they may be effective for those applications. The typical uses
listed for each treatment are provided to indicate applications for which the treatment is known
to have been used. The documented effects are primarily based on these known applications. The
lists of typical uses are helpful when considering driver expectations associated with a treatment.
3.5.2 High-Speed Intersection Applications
The high-speed intersection applications provided in Section 4 describe high-speed intersec-
tion conditions for which a treatment may be well suited. This section also describes conditions
for which the treatment is not well suited. For example, dynamic warning signs may be appro-
priate on approaches to rural unsignalized intersections, but may not be appropriate in contexts
where there is already a high driver workload or in a complex visual environment where the
dynamic warning signs may prove ineffective.
3.5.3 Function
The function(s) of the treatments describe the role that each treatment plays in the intersec-
tion environment to produce lower speeds. The treatments should be screened to ensure that
those selected have functions that are suited to the specific speed problem(s) identified in the
intersection assessment. For example, if speed adaptation is identified as the root of a speeding
problem, it is likely that a treatment that reinforces an impression of changes in the character of
the roadway environment sufficiently in advance of the target speed location will be most suc-
cessful in reducing speeds. Similarly, if driver attentiveness is identified as the likely root of the
problem, appropriate treatments might be those that alert drivers to the upcoming intersection.
3.5.4 Documented Effects
There is limited documented research that identifies how much speed reduction (if any) may
be expected from a given treatment. Most available documentation relates to segment speed
reduction applications rather than intersection speed reduction. However, these data are
included because it is logical to assume that, in many cases, similar effects may be achieved on
high-speed intersection approaches. Data that specifically address speed reduction effectiveness
on high-speed intersection approaches are available for the following treatments: dynamic warn-
ing signs, transverse pavement markings, roundabouts, approach curvature, and rumble strips.
Because the extent of these data is quite limited, the speed reduction actually realized will likely
vary with each application. The information available related to each specific treatment's effec-
tiveness is provided as part of the detailed treatment descriptions.
3.5.5 Cost
Cost is often a factor that limits treatment implementation. In general, treatments that do not
involve changes to the paved roadway section have the lowest costs, while treatments that mod-
ify or reconstruct the roadway section are the most costly. Secondary costs such as establishing
a power source in a rural location could also increase implementation costs.
3.5.6 Experimental Status
Identifying whether a treatment would be considered experimental--whether or not it is
included in the MUTCD--helps the user determine if additional authorization may be needed
before an experimental treatment can be installed.
