WELCOME

1. WELCOME Signal Timing for Congested Conditions: The Web seminar will begin at 12 noon EDT.

2. HOUSEKEEPING • Synchronize your audio and web connections. • All participant phone lines are muted. • Questions may be asked via the Chat Room. • If you have technical difficulties dial 1-800-305-5208 to contact the Genesys help desk.

3. EARNING CEU AND/OR PDH Successful completion of this Web seminar includes: • Verification of attendance • Completion of course evaluation • Verification of learning objectives (online quiz) These requirements must be met to earn 1.5 PDH or .15 IACET CEU. At the conclusion of the course you will receive an email with directions to the online quiz. An additional fee may apply.

4. INSTRUCTOR Philip J. Tarnoff Director Center for Advanced Transportation Technology tarnoff@eng.umd.edu

5. Signal Timing for Congested Conditions

6. Course Objectives • Analyze field conditions to determine the types of congestion that exist • Identify signal timing countermeasures that can be applied to each type of congestion • Define extreme measures that might be used to ease high levels of congestion • List measures other than signal timing that might be applied to further reduce high levels of congestion

7. Agenda • Characteristics of congestion • Brief review of relevant signal timing concepts • Timing plans for congested conditions

8. Characteristics of Congestion • Cycle Failure – Queues not completely discharged during green phase • Demand – Ratio of demand to capacity greater than one (over-saturation) • Intersection demand exceeds capacity • Approach demand exceeds capacity • Characteristics - May spread from a single location or facility • Transient Condition – Begins at a location, spreads, and then dissipates

9. Congestion vs. Saturation

10. Under-Saturated vs. Saturated • Saturated conditions characterized by changing circumstances (growing queues) • Under-saturated conditions characterized by constant conditions (constant queue lengths) • Measures are different • Under-saturated – minimize stops and delays, maximize bandwidth • Saturated – avoid spillback, prevent turn bay overflows, minimize duration of problem • Signal timing software is designed for under-saturated conditions

11. Agenda • Characteristics of congestion • Brief review of relevant signal timing concepts • Timing plans for congested conditions

12. Signal Timing Fundamentals Brief Review • Cycle – Time required for signal to display a complete sequence of colors • Split – Time allocated to a given movement relative to cycle length • Offset – Start of cycle at one intersection relative to start of cycle at adjacent intersection

13. Conventional Approach to Saturation • Cycle – Increase to highest tolerable value • Split – Impose long delays on side streets • Offset – Some use simultaneous offset, others reduce speed of progression Are these the correct approaches?

14. Signal Timing Fundamentals Cycle Split (A phase) Offset

15. Time-Space Diagram Space Bandwidth Speed Time

16. Cycle Length – Fact and Fiction • Fact: • Increased cycle length increases intersection capacity • Shorter cycle lengths reduce delay • Fiction: • Cycle length has a significant impact on capacity • Offset and cycle length are independent • Longer cycle lengths always reduce congestion

17. Cycle Length and Capacity

18. Cycle Length change 2 Phase 3 Phase 4 Phase 120 to 140 0.75% 1.16% 1.59% 140 to 160 0.56% 0.86% 1.17% 160 to 180 0.43% 0.66% 0.90% 180 to 200 0.34% 0.53% 0.71% Cycle Length and Capacity

19. Longer Cycle Lengths CanIncrease Congestion • Upstream throughput may exceed downstream link capacity • Turning bay storage can be exceeded • Increased vehicle headways with long cycle lengths (longer green times)

20. Long Cycle Length Reduces Capacity Space Lost (wasted) green time Time

21. Cycle Length and Headway Space Headway Time

22. The “Bottom Line” On Cycle Length Results for an Isolated Intersection

23. Basic Facts About Split • Allocates intersection capacity to conflicting movements • Directly entered on pretimed controllers • Implicitly selected for actuated controllers through: • Maximum green times • Minimum green times

24. Split Is Most Important At High Demand

25. Conclusions About Split • For under-saturated and some saturated conditions: • Split i = Cycle × (Volumei)/(Total Volume) • For some saturated conditions, split may be used to: • Meter incoming traffic • Encourage diversion

26. Offsets Can Produce Smooth Flow • Offset is the time relationship between intersections • Offset effectiveness limited for saturated conditions • Offset is useful for higher intersection spacing than generally realized

27. Why Do I Need to Know All This? • Signal timing optimization programs do NOT always provide the best signal timing • Designed for steady-state conditions • Cannot optimize for oversaturation • Do not model mid-block sources well • Do not permit evaluation of alternative phase sequences • Engineering analysis is mandatory

28. Summary of Timing Considerations • Longer cycle lengths are not necessarily better • Incorrect splits (including maximum greens) can significantly increase delay • Offsets are more important for under-saturated intersections

29. Questions ???

30. Agenda • Characteristics of congestion • Brief review of relevant signal timing concepts • Timing plans for congested conditions

31. Signal Timing Process Arterial Timing Saturated? Select Cycle Length Range Select Cycle Length Range no yes Use Optimization Software Evaluate Phasing Alternatives Determine Offsets and Splits

32. Identifying Saturation • Growing queues • Cycle failures • Turn lanes blocking thru movements • Occupancy > 25%

33. The Process Depends on Whether Saturation Exists Section Saturated? no yes Computer Optimization Manual Techniques

34. Definitions • Critical Intersection (CI) – An intersection whose capacity is the limiting value of a segment of roadway or the entire system • Congestion – Traffic demand approaches or exceeds the capacity of a signal • Saturation – Queues form but their effects are local • Over-saturation – Queues fill entire blocks and interfere with adjacent intersection operation

35. Types of Congestion • Type I – CI has less green time than upstream intersections (typical of CI at junction of crossing arterials) • Type II – CI has lower capacity due to additional phases, geometrics, or backups from downstream intersection • Type III – CI has greater demand due to heavy turning at upstream intersection • Type IV – Congestion due to offset relationship with upstream intersection

36. Discussion • Are these the correct categories of congestion? • If not, what changes would you make? • What measures might be applied to each form of congestion?

37. For Congestion on a Link • First determine CI and type of congestion • Determine whether a split adjustment will solve the problem (Types I, II) • Determine whether the cycle length is too long for the block length (Type II) • Determine whether the offset is wrong (Types III, IV) • Identify blockages to flow – double parking, parking garages, driveways, etc. (Type II)

38. Split Adjustment • Problem is most likely incorrect splits due to actuated controller MAX settings • Splits should be based on critical lane flows and not total approach volumes • Consider split that favors “problem” flows

39. Cycle Length • Use maximum cycle length that is: C < (1/R) × (3600/F) × (L/V) Where: F = Critical lane volume on critical phase (vph) L = Link length of approach on critical phase (ft) V = Vehicle length + separation (ft.) = (20 ft.) R = Red split of critical phase (%)

40. Offset • Offset may be negative (downstream goes green before upstream) Offset = (L/S – Q/R) Where: L = Link length (ft.) S = Speed (ft./sec.) Q = Queue (vehicles/lane) R = Discharge rate (veh./lane/sec.) (about .5)

41. Other Measures • Eliminate driveways near intersections • Eliminate parking • Enforce double parking regulations • Move bus stops • Reduce phases by banning left turns (may require alternative routes for left turners) • Change phasing • Increase length of turn bays

42. Extreme Measures - Metering • Applicable to Types I, II and III • Concept - Avoid upstream spillback, intersection blockage and overflow of turn bays • Applied by reducing inflow to CI by: • Adjusting upstream timing (cycle or split) • Reducing side street splits to reduce turning traffic entering CI approach

43. Extreme Measures – Equity Offsets • Concept – Calculate offsets to ensure that upstream turning traffic does not take too much green time at the CI • Calculate vehicles of turning traffic to be permitted in link for each cycle • Calculate offset between CI and upstream intersection so that the distance between end of the queue and intersection is adequate for storage of vehicles • Best determined by simulation or trial and error

44. Extreme Measures – Equity Splits • Concept — All congested directions “share the pain” — Spillback minimized • Apportion split so that rate of growth in congestion equalized in all congested directions • Best determined by simulation or trial and error

45. Extreme Measures – Influence Travel Behavior • Extremely long delays on side streets will influence side-street demand (Examples of 5 min. delays exist) • Encourage trip distribution (routes) by imposing excessive delays on selected arterials (divert traffic from critical intersections) • Encourage transit usage through exclusive lanes and signal preemption • Encourage transit usage, deferred travel, etc. through traveler information and public relations campaigns (useful during major construction)

46. Final Thoughts • Timing for saturated conditions is VERY different from timing for under-saturated conditions • Timing must consider cause of congestion • Objectives are to limit the duration of the congestion and control its spread

47. BEFORE YOU GO… • Remember to submit sign-in sheets and evaluation forms • Online quiz information will follow in an email to course registrants. The quiz must be taken within two weeks of the course. Questions/Comments Aliyah N. Horton Senior Director Professional Development and Outreach ITE 1099 14th St., NW, Suite 300 West Washington, DC 20005 202-289-0222 ext. 137; ahorton@ite.org