1. CE 2710: Transportation Engineering Traffic Signals April 3, 2009 Nicholas Lownes, Ph.D.

2. Traffic Signals – Why? Increase throughput Reduce delay Improve safety Provide progression through network Help low volume roads Source: www.flickr.com/photos/meckleychina/926962569/

3. Current System “It is estimated that improper traffic signal timing accounts for 5 to 10 percent of all traffic delay, or 295 million vehicle-hours of delay, on major roadways alone.”

4. What do they Cost? Installation Cost: $75,000 - $500,000 –Signal heads –Support structure (cable or cantilever) –Sensors –Wiring –Controllers –Labor

5. What do they Cost? Annual Maintenance: $3,000 - $8,000 –Labor –Bulb replacement Electricity: ≈$1,400/yr (Arizona DOT) Update Signal timing: $2,500 - $3,100 per intersection per update –Should be done often – cheap way of improving operations –Labor-intensive

6. What do they Cost? Upgrade signal: $10,000 per intersection –Should be done every 10 years (National Traffic Signal Report Card) Major Investment… not to be taken lightly. Always consider alternatives.

7. Alternatives Improve markings, signage, other (less expensive) control devices If safety is concern: –use speed mitigation measures –Roadway lighting (if nighttime is major issue) Restrict turning movements Add turning lanes

8. When is a Traffic Signal Warranted? Manual on Uniform Traffic Control Devices (MUTCD) –Available free online: http://mutcd.fhwa.dot.gov http://mutcd.fhwa.dot.gov National Minimum Standard for all traffic control devices on streets, highways and bicycle trails. VS.

9. Signal Warrants – Volume #3 Peak hour (1 hour on average day) 465

10. Sidebar How do you determine an average day? –Which day of the week? –What month of the year? –What type of weather?

11. Signal Lingo Phase: allotted time to a movement Ring: Sequence of phases –Green phase (G) –Red phase (R) –Amber (yellow) phase (Y) –All-red phase (AR) Cycle length (C) – time from start of green phase to green phase on an approach (typically 45-180 s) C = G + R + Y + AR

12. NEMA Ring & Barrier Structure www.tfhrc.gov/safety/pubs/04091/04.htm

13. Permissive Lefts Only (2 phase) www.tfhrc.gov/safety/pubs/04091/04.htm

14. Split Lefts (6 phase) Split timing not usually the most efficient Protected Left and Right Permissive turns Split phases

15. Dual Lefts Papacostas & Prevedouros, 1993 Leading Dual Protected Left Turns Pedestrian Movements

16. Common Question How do pedestrian push-buttons work? Pedestrian Volume “Assertiveness” Interrupt Awareness For Show Note: Oval size approximates the frequency of each type in practice

17. Goals of Signal Timing Minimize the # of Green phases Maximize the # of vehicles moving through the intersection during all green phases –Keep as many traffic streams flowing at all times as possible Minimize delay (shorter cycle lengths) Maximize throughput (longer cycle lengths)

18. Types of Signals Pretimed Semi-actuated Fully Actuated Adaptive

19. Pretimed Lengths of Phases predetermined and statically set Strengths –Simplest –Less infrastructure (read: maintenance) Weakness –Can not account for cycle-to-cycle variation of traffic –Will remain fixed until updated (which costs $)

20. Critical Movements Left turns and right turns take longer –Protected left turns take a factor of roughly 1.6 times as long as a through movement –Unprotected lefts can take up to ten times as long (depending on opposing traffic) –Right turns take roughly 1.4 times as long Critical movements are approach-by- approach –The maximum converted lane volume

21. Example 460 vph (T) 340 vph (T) 120 vph (R) 820 vph (T)640 vph (T) For SB approach, critical volume is max{820, 640} = 820 For EB approach, critical volume is max{460, 340+120*1.4} = 508

22. Cycle Length Rule of Thumb ≈ 45 – 180 s Webster’s Method (min delay) Optimal Cycle Length Lost Time = yellow + all-red Critical volume for phase i Saturation flow

23. The Highway Capacity Manual (HCM) 2000 suggests 1900 vphpl as the base saturation flow rate This baseline can be increased or decreased depending upon the situation Factors such as: –Lane width –Grade –Pedestrians –On-street parking

24. Example C 2 = 508 C 1 = 820 Assume: 2 phases, 3s Yellow for each

25. Allocating Green Time Minimum Pedestrian crossing phase Need to make sure our green phases are at least 10 s 7 second baseline from MUTCD – min walk interval 12’ lanes 4 ft/s peds

26. Green Allocation Allocate Green time proportional to critical movement Total Green time = C o – L = 41 s

27. Progression Moving through a series of signals without stopping Certain assumptions generally apply –You drive the speed limit (or the design speed) –Uncongested traffic Source: http://www.minagarinc.com/red.jpg

28. Progression Assumptions can be released, but it gets much more complex Offset = time increment from appearance of base intersection green to green at intersection of interest –Offset should be multiple of ½ cycle length (for manual method at least)

29. Progression Bandwidth = length of time in which one could arrive at intersection and achieve progression –Increases with cycle length –Decreases with increase in progression speed –Decreases with queue clearance

30. Distance (ft) Time (Cycles) Intersections Draw initial sloping line with slope ½ cycle per 1000 ft For Intersections, choose offset of 0 or ½ cycle placing beginning of green as close to sloping line as possible 1000’ 2200’

31. Distance (ft) Time (Cycles) Intersections Then adjust line as needed

32. Distance (ft) Time (Cycles) Intersections Repeat

33. Distance (ft) Time (Cycles) Intersections Repeat

34. Distance (ft) Time (Cycles) Intersections Repeat

35. Distance (ft) Time (Cycles) Intersections We’ve now determined offsets for each intersection

36. Actuated Signals Most use Inductive-Loop Detectors in pavement Source: FHWASource: www.richmond.ca/__shared/printpages/page2080.htm

37. Fully actuated Sensors/detectors on all legs of intersection There is a pretimed framework that underlies an actuated intersection Green Phase on an approach is requested (or extended) if presence of vehicle is detected. Can enter the next phase in one of two ways:  Max out: the phase reaches its predetermined maximum  Gap out: no vehicle detected for a movement within some predetermined amount of time

38. Max Out –Example Green Phase Green Extension = 4s Time, t (s) Green time remaining (s) Vehicle arrives at t = 4, 5, 7, 16, 18 s 20 Not to scale 10 Minimum Green = 10s Max Green = 20s AMBER RED

39. Gap Out –Example Green Phase Green Extension = 4s Time, t (s) Green time remaining (s) Vehicle arrives at t = 4 s 20 Not to scale 10 Minimum Green = 10s Max Green = 20s Actual Green Phase = 14s AMBER RED

40. Summary Signals are a major investment and their installation requires careful thought We want to maximize throughput and minimize delay We want to provide progression for signalized corridors Pre-timed signals are simple & cheaper Actuated can account for cycle-to-cycle variation

41. Summary Pre-timed –Lower maintenance resources –Consistent demand –Simpler Actuated –Significant variation in demand –High volume meets low volume road –Greater control