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RW Item No. 5
National Committee on Uniform Traffic Control Devices
TASK FORCE RECOMMENDATION
TECHNICAL COMMITTEE: NCUTCD Regulatory/Warning Signs Technical
Committee
DATE OF ACTION: (TASK FORCE) 12-8-08, revised 12-9-08, revised 12-22-08
RWSTC APPROVAL DATE:
TRANSMITTAL TO SPONSORS DATE:
COUNCIL APPROVAL DATE:
TOPIC: Approaches to Signalized Intersections in a 50+ speed zone - 85th Percentile
Speed, Section 2B.13, 2003 MUTCD and NPA language
AFFECTED PORTIONS OF MUTCD: Section 2B.13 of 2003 MUTCD and NPA
Problem Statement: Is the use of the 85th percentile speed shown in Section 2B.13
appropriate for higher speed approaches (50+ MPH) to signalized intersections?
Background: Section 2B.13 of the NPA and recommended language changes approved
by NCUTCD in June 2008 states:
Standard:
Speed zones (other than statutory speed limits) shall only be established on the basis of
engineering study that has been performed in accordance with traffic engineering
practices. The engineering analysis shall include an analysis of the current speed
distribution of free-flowing vehicles.
Guidance:
When a speed limit within a speed zone is posted, it should be within 10 km/h or 5 mph
of the 85th percentile speed of free-flowing vehicles.
Research shows that crash rates go down when speed limits are within 5 MPH of the 85th
percentile speed. (see chart below)
Should transitional speed zones be used approaching a signalized intersection?
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Generally, it is not recommended to have transition/step down speed zones. Transitional
speed zones are normally relatively short in length and provide a means to allows drivers
to slow down their speed when approaching zones that are reduced due to constraints
such as urban areas, approach to a signal, or approach to a STOP controlled intersection
or approach to a roundabout. Research suggests that drivers may not reduce their speed
to the posted speed limit on the basis of signage alone. Speed is more dependant on other
factors, such as the physical characteristics of the highway or in the case of a signal; a
change in phase from green to yellow or yellow to red. Historically, speed data has been
analyzed in transitional zones. It resulted in increased dispersion of variance of
individual speed. The probability of collisions increased with speed variance.
Question and Recommendation: Therefore, is there a need to reduce the regulatory speed
limit on a roadway in the vicinity of isolated intersections controlled by a traffic control
signal? In this situation, the section of highway within approximately one-half mile of the
intersection is generally considered.
Rather than promoting artificial restrictions in advance of a condition, proper design or traffic
control signing and marking features of the intersection control will be required to address
the safety & operational needs of the signalized approach. These typically include:
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Intersection lighting
Advance Signal Ahead Warning Sign (W3-3)
Beacons on advanced signing as appropriate
Double marking the W3-3 as appropriate
Be Prepared to Stop Advanced warning sign (W3-4)
Dilemma Zone Detection for high speed approaches
Speed limits need to be reasonable and enforceable. Just as speed-zoning criteria is
considered, any section of roadway on approach to a signalized intersection should be treated
in similar regard.
Rather than reducing the speed limit arbitrarily in advance of a signalized intersection
stemming from safety concerns, improvements should be considered that pertain to the
installation/location itself (e.g. channelized turning movements, extended turn bays,
modification to signal phasing or timing, rumble strips, advance warning signs, warning
beacons, signing/marking enhancements, etc.).
Speed limit reductions in advance of the installation will likely not influence safety at the
condition and may even promote poor engineering decisions in the future since certain
functions, such as signal timing or sign placement, can be based on posted speeds.
Accordingly, speed studies for approaches to signalized intersections should be taken outside
the influence area of the signal to avoid skewed results related to the 85th percentile speed.
Typically, this is one-half mile.
Recommended change to MUTCD, Section 2B.13 –
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Guidance: Speed studies for approaches to signalized intersections should be
taken outside the influence area of the signal to avoid skewed results related to
the 85th percentile speed.
Support: Advance warning signs and other devices to attract the motorists’ attention
to a signalized intersection are usually more effective than a reduced speed zone.
u: signal in a 50+ MPH zone RWSTC 12-8-08, revised 12-9-09
Vote:
For:
Opposed:
Absetentions
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