NCHRP 3-67: Expert system for
recommending speed limits in
speed zones
Raghavan Srinivasan, Ph.D.
David Harkey
Dwayne Tharpe
UNC Highway Safety Research Center
Martin Parker
Wade Trim Associates
Roy Sumner
PB Farradyne
HSIS Annual Meeting, 10/2006
Objective of the Project
• Develop a knowledge-based expert system for
recommending speed limits (web-based
application)
• Speed limits need to be enforceable and credible
to:
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Traffic engineers
Elected officials and public policy makers
Drivers and citizens living and working in the area
Judges and magistrates
Enforcement officials
HSIS Annual Meeting, 10/2006
History of Expert Systems for Speed
Limits
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Developed in Australia starting in the 80’s
VLIMITS – VicRoads
NLIMITS – New South Wales
SALIMITS – South Australia
WALIMITS – Western Australia
QLIMITS - Queensland
TLIMITS - Tasmania
NZLIMITS – New Zealand
USLIMITS – USA (Developed by ARRB)
– Accessed through Internet; Username & Password can be obtained
from Davey Warren at FHWA
• USLIMITS2 – USA (Developed in NCHRP Project 3-67)
HSIS Annual Meeting, 10/2006
Project Scope
• All types of roadways from rural two-lane
segments to freeway segments
• The following are not included:
– Roadways segments controlled by statutory
limits
– Roadway segments controlled by temporary
and part-time limits, e.g.,:
• Work zones
• School zones
HSIS Annual Meeting, 10/2006
Approach
• Identify panel of experts
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Traffic engineers
Enforcement personnel
One judge
One decision maker
Two researchers
• Using case studies, determine critical factors
• Conduct meetings with expert panel to finalize the
decision rules
HSIS Annual Meeting, 10/2006
Case Studies
• Web-based case studies
– Case studies were implemented on the web
– Each case study had several scenarios
– Within each case study, one factor was modified while
keeping other factors constant
– Provided with hypothetical site characteristics data and
a photograph
– Asked to identify the critical factors, and provide the
appropriate speed limit
• A pilot was implemented in Spring 2005 (20
respondents)
• A more detailed survey was implemented in Fall
2005 (44 respondents)
HSIS Annual Meeting, 10/2006
Meetings
• June 2004 with expert panel
– Obtained preliminary information on critical factors
• August 2004 with NCHRP panel
– Presented our approach
– Panel recommended research team to conduct surveys
using case studies
• December 2005 with expert panel
– Presented results of case studies
– Made decisions regarding the decision rules
• Could not get a consensus on some issues; research team had
to use its judgment on these issues
HSIS Annual Meeting, 10/2006
Overview of USLIMITS2
• Users create their own username and
password
• For any project, users indicate if it is a:
– Limited Access Freeway
– Road Section in Undeveloped Area
– Road Section in Developed Area
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Residential Subdivision/Neighborhood Street
Residential Collector Street
Commercial Street
Street Serving Large Complexes
HSIS Annual Meeting, 10/2006
Factors in USLIMITS2
• Common Variables
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Operating Speed (85th and 50th percentile speeds)
Presence/absence of adverse alignment
Is this is a transition zone?
Section length
AADT
Crash Statistics
Average crash and injury rates for similar sections
• If this information is not available, average values for different
roadway types and AADT categories based on data from HSIS
States is used
– Statutory limit for this type of road
HSIS Annual Meeting, 10/2006
Additional Factors in USLIMITS2
• Limited Access Freeway
– Number of interchanges in the section
• Road Section in Undeveloped Areas
– Roadside hazard rating
– Divided/Undivided
• Road Section in Developed Areas
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Extent of ped/bike activity
Presence/usage of on-street parking
Number of traffic signals
Number of driveways and unsignalized intersections
HSIS Annual Meeting, 10/2006
Decision Rules
• Program calculates speed limit with and without
crash statistics and selects the lower value
• Speed limits typically between the 85th (rounded to
the nearest 5 mph) and the 50th percentile
(rounded to the nearest 5 mph)
• Warnings for different situations
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If speed limit is higher than statutory value
Length is shorter than minimum length
There is adverse alignment
Crash or injury rate is higher than critical rate or at least
30% higher than average rate
– 85th percentile seems too high for a particular roadway
type
HSIS Annual Meeting, 10/2006
Other Features
• Users are provided with
– User Guide
– Flow charts describing the decision rules
– Study report describing the project
• Project will be completed in November 2006
• Will be hosted by HSRC for about an year
(www2.uslimits.org)
HSIS Annual Meeting, 10/2006