Variable Speed Limits
Making Work Zones Work Better Workshop Orlando, Florida 17 Sept. 03
Davey WarrenOffice of Safety ProgramsFederal Highway Administration
Presentation Overview
• What is a variable speed limit?
• How do variable speed limits work?
• Objectives of variable speed limit system
• Work Zone Tests
• Lessons learned
• Future products and additional information
What is a Variable Speed Limit (VSL)?
• Speed limit changes with changing conditions
19501950
Part-time Speed Limit Work Zone
Focus of presentation
• Modern systems change the speed limits in real time:• Traffic conditions
• Adverse weather conditions
• Road surface conditions
• Work Zones
19601960
Computer controlled limits
Variable Speed Limit System Components
• traffic and speed detectors
• variable speed signs
• microprocessor
• communication
• environmental sensors
• base station for recording speed limit changes
Why Use Variable Speed Limits?
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12 14 16 18 20 22 24TIME
Spe
ed,
mi/h
Objectives
• Increase compliance
• Improve safety
• More efficient use of highway
• Less burdened justice system
• Responsive to dynamic conditions
• Provide real time information
Variable Speed Limit Applications
• General VSL
• Winter Weather and Adverse Road Conditions
• Fog
• Congestion
• Work Zones
General VSL
Examples• NJ Turnpike
• I-40 New Mexico
• Germany
Characteristics
• Typically cover longer stretches of roadway
• Broad range of input criteria for speed limit decision (traffic speed, volume, crashes, congestion, construction, ice, snow, fog, etc.)
I-40 New Mexico
• fully automated
• maximum speed limit• constrained by NMSL
• minimum speed limit
• downstream hazard warning
• roadside station after each interchange
20
30
40
50
60
70
80
0 2 4 6 8 10 12 14 16 18 20 22 24
TIME
Sp
ee
d,
mi/h
Smooth speed Speed Limit Speed Limit With Histeresis
Average Speeds and Variable Limit Over 24 hr
N.M. Automated Speed Control Logic
Smooth Mean Speed +/-
Congestion
• Examples
Lodge Freeway, MI
M25, UK
Netherlands
• Characteristics• Cover stretches of congested roadways
• Speeds set to reflect traffic conditions
• Slow traffic approaching backups
19601960
Winter Weather and Road Conditions
• ExamplesI-90, Washington
E18, Finland
I-40, Arizona
• Characteristics• Cover longer stretches of weather-susceptible
roadways
• Speeds set to reflect roadway/weather/visibility conditions
Fog
• ExamplesI-75, Tennessee
I-80, Nevada
F-6, Australia
A 16, Netherlands
• Characteristics• Typically deployed in areas
that experience highly variable, severe fog
• Speed and visibility sensors
VSL in Work Zones
• Dynamic conditions in work zones make them excellent candidates for VSL
• FHWA is sponsoring 3 field tests of VSL in work zones
• Michigan
• Maryland
• Virginia
Field Test Objectives
• Objectives:• Deploy practical variable speed limit
systems in work zones
• Evaluate the effectiveness of the systems on:
• Speed limit compliance• Credibility of speed limits• Improved Safety• Improved Traffic flow
Michigan’s VSL System
• Four deployments to date
• Where: • 19-mile work zone
• I-96 near Lansing
• ADT 29k – 53k
• (over 10% trucks)
• When: late May – August 2002
• Duration: Short – All less than 1 week
I-96 Michigan
• 6-7 variable speed limit trailers• RTMS Traffic detector
• Solar power
• Controller with RF communication
• LED speed display
• ½-1 mi spacing• line of sight communication
• 70 to 40 mi/h• Based on prevailing speed
• Max based on nature of road work
• Pagers used to transmit speed limit changes to police
Michigan’s Speed Control Logic
Profile 1 Workers Unprotected
2 Workers Present
3 No Work Activity
L.O. Threshold - 0% 50 60 70
H.O. Threshold – 70% 40 40 40
v < 40 40 40 40
40 ≤ v < 43 45 45 45
43 ≤ v < 48 50 50 50
48 ≤ v < 53 50 55 55
53 ≤ v < 58 50 60 60
58 ≤ v < 63 50 60 65
63 ≤ v < 68 50 60 70
v ≥ 68 50 60 70
Michigan’s VSL System
• Preliminary Results:
• Better compliance
• More uniform speeds during off-peak
• Travel times reduced
• Greater speed reduction at crossover
• RTMS accurate for volume but marginal for speed
• Trailers can be set-up within 10 minutes
Maryland’s Work Zone VSL
• VSL Trailers – 4
• Radar unit for speed
• Queue Detector Trailers – 2
• Variable Message Signs on Trailers – 2
• Base Unit – Central Control (CHIPS System Trailer) – 1
Maryland’s VSL Algorithms
• Logic:• User selectable percentile speed
• Percentile can vary depending on whether workers present or not
• User can specify times to switch percentile
• Speed limit will be updated no more frequently than every 2 minutes.
• 2 other candidate algorithms programmed• Tweener
• Oz
Maryland’s VSL System Status
• Status:
• Completed pre-deployment testing and accepted equipment
• First deployment will occur in the next month
• The second deployment has yet to be scheduled
Lessons Learned
• Engineering• Perform “off-site” testing on a real road
• Eliminate unnecessary fluctuations in limit
• May need to change as often as once a minute
• Need signs on both sides of road with 3 or more lanes
• Implementation• Make sure that all stakeholders are on board and working
towards common goals
• Don’t try to do everything all at once – start with something easy
• Work closely with the media; have only 1 or 2 PR people for the media to contact
• Be flexible (expect the unexpected)
Summary
• VSL being used around the world to help manage speed and improve safety
• VSL seems to be particularly appropriate for work zones
• Enforcement remains an issue
• More and better data needed on effectiveness
Additional Informationhttp://www.itsdocs.fhwa.dot.gov/itsweb/welcome.htm• Examples of Variable Speed Limit Applications (EDL#12164)
• Safety Applications of ITS in Rural Areas (EDL#13609 )
http://gulliver.trb.org/publications/nchrp/nchrp_lrd_47.pdf• NCHRP Legal Research Digest 47: Judicial Enforcement of Variable
Speed
http://safety.fhwa.dot.gov/fourthlevel/pdf/poynton1.pdf• Controlled Motorways: Variable Speed Limits on the M-25, UK
http://safety.fhwa.dot.gov/fourthlevel/ppt/Warren_Vsl.ppt• Variable Speed Limit slide show
Future Products• NCHRP 3-59 Variable Speed Limit Implementation Issues (2004)
• Field Test Evaluation Cross-Cutting Study Report (2004)
Questions
Davey Warren
FHWA HSA-20
400 7th Street, SW
Washington, DC 20590
202-366-4668
Recommended
