Variable Speed Limits HUMAN FACTORS WORKSHOP ON WORK ZONE SAFETY: PROBLEMS AND COUNTERMEASURES Washington, DC

21 January 2007

Davey Warren Office of Safety Design Federal Highway Administration

Presentation Overview • What is a variable speed limit? • Why variable limits in work zones? • Work zone applications • Issues and challenges

What is a Variable Speed Limit (VSL)? • Speed limit changes with conditions

1950

Why VSL in Work Zones?

Why Variable Speed Limits? • Work zones are dynamic • Safe speed varies with conditions • Static limits only good for condition set 80 70 60

Speed, mi/h

50 40 30 20 10 0 0

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4

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12 TIME

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Work Zone Application- New Mexico

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Work Zone Application – New Hampshire

ITS Application • Modern systems change the speed limits in real time: • Traffic conditions • Adverse weather conditions • Road surface conditions

• Work zone activity

Computer controlled limits

Variable Speed Limit Components • traffic and speed detectors • microprocessor • communication • variable speed signs • base station for logging changes • environmental sensors

Variable Speed Limit Display • Typically displayed on roadside or overhead; on trailers in work zone • Signs may be permanent or temporary, depending on application

Work Zone Application - Michigan • 19 mile work zone • 4 deployments of short duration • 6-7 monitoring and display trailers • RTMS traffic detection unit • Solar power • Controller with RF communication • ½ -1 mile spacing

• 70 to 40 mi/h • Based on prevailing speed; 6 min update • Max based on nature of road work

• Speed limit changes transmitted to police via pagers

Michigan’s Speed Control Logic Profile

1 Workers 2 Workers 3 No Work Unprotected Present 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

Static 50

start time Thu., 05/23/02 Thu., 06/06/02

Fri., 05/24/02 Fri., 06/07/02

23:00

22:00

21:00

20:00

19:00

18:00

17:00

16:00

15:00

14:00

VSL

13:00

100 90 80 70 60 50 40 30 20 10 0

12:00

% of vehicles w/speed > 60 mph

Speeds of fastest driver decreased

Michigan’s VSL System • Results: • • • • • •

Better compliance More uniform speeds during off-peak Travel times reduced Speed of fastest drivers decreased Accidents lower in direction with VSL RTMS accurate for volume but marginal for speed • Trailers can be set-up within 10 minutes

Work Zone Application - Maryland • VSL Trailers – 4 • Radar for speed • White LED • Isolated or linked

• Queue Detector Trailers – 2 • Variable Message Trailers • Base station – web based

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

Human factors issues & challenges • Display color • Update frequency • Magnitude of posted speed change • Transition zone or notice of reduced speed ahead • Location of signs • Sign spacing

Color of displayed speed • White or yellow on black background?

Frequency of update • 1, 2, 5, 10, 15 minutes? 80 70 60

Speed, mi/h

50 40 30 20 10 0 0

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4

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12 TIME

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Eliminate unnecessary SL changes 80 Smooth speed

Speed Limit

Speed Limit With Histeresis

Speed, mi/h

70 60 50 40 30 20

0

2

4

6

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10

12 TIME

14

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24

Magnitude of speed drop • From one time period to next • 5 mph, 10 mph, 15 mi/h, whatever?

• From one VSL trailer to the next • Reduced speed ahead needed? 80 70 60

Speed, mi/h

50 40 30 20 10 0 0

2

4

6

8

10

12 TIME

14

16

18

20

22

24

Location of signs • When are VSL signs needed on both sides of road? • 3 or more lanes? • Large % trucks?

VSL repeater sign (trailer) spacing • Maximum distance between VSL trailers • ¼ mi, ½ mi, 1 mi, 3 mi, 5 mi ?

Enforcement • Certified documentation of posted speed • Technology assisted speed enforcement • Pagers, internet

• Automated enforcement

Summary • VSL being used around the world to help manage speed and improve safety • VSL seems to be particularly appropriate for work zones • Frequency of speed display changes in time in space are HF issues • Enforcement remains an issue • More and better data needed on effectiveness

Additional Information http://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 Limits

http://safety.fhwa.dot.gov/fourthlevel/pdf/poynton1.pdf • Controlled Motorways: Variable Speed Limits on the M-25, UK http://safety.fhwa.dot.gov/speed_manage/docs/warren_vsl.ppt • Variable Speed Limit slide show

Future Products • NCHRP 3-59 Variable Speed Limit Implementation Issues (2007) • Field Test Evaluation Cross-Cutting Study Report (2007)

Questions Davey Warren FHWA HSA-10 400 7th Street, SW Washington, DC 20590 202-366-4668 [email protected]

http://safety.fhwa.dot.gov/