CE 2710: Transportation Engineering

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 • 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 Pedestrian Movements LeadingDualProtected Left Turns Papacostas & Prevedouros, 1993

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

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

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

23. Saturation flow • 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 Assume: 2 phases, 3s Yellow for each C1 = 820 C2 = 508

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

26. Green Allocation • Allocate Green time proportional to critical movement • Total Green time = Co – 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. For Intersections, choose offset of 0 or ½ cycle placing beginning of green as close to sloping line as possible Draw initial sloping line with slope ½ cycle per 1000 ft Time (Cycles) 1000’ Intersections Distance (ft) 2200’

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

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

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

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

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

36. Actuated Signals • Most use Inductive-Loop Detectors in pavement Source: FHWA Source: 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 Not to scale Max Green = 20s Minimum Green = 10s Green time remaining (s) AMBER RED 10 Green Extension = 4s Time, t (s) 20 Vehicle arrives at t = 4, 5, 7, 16, 18 s

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

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