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Traffic Impacts of Short Term Interstate Work Zone Lane Closures: The South Carolina Experience

Investigating the impacts of short-term interstate work zone lane closures in South Carolina, with a focus on traffic flow theory, measurement techniques, and capacity analysis.

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Traffic Impacts of Short Term Interstate Work Zone Lane Closures: The South Carolina Experience

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  1. Traffic Impacts of Short Term Interstate Work Zone Lane Closures: The South Carolina Experience Wayne Sarasua, Ph.D., P.E. sarasua@clemson.edu Making Work Zones Work Better

  2. Overall Goals • Develop a means to determine an actual volume of vehicles per hour per lane to be used to determine when lane closures may be permitted. • Develop a means to estimate speeds, delays, and queue lengths due to short-term lane closures in work zones Making Work Zones Work Better

  3. Overall Goals (Cont’d) • Analyze the effects of roadway grades, truck percentages, and lane widths on work zone traffic characteristics when lane closures are present Making Work Zones Work Better

  4. Research Participants • Clemson University • Wayne Sarasua • David Clarke • Several GRAs • The Citadel • William J. Davis Making Work Zones Work Better

  5. Knowledge Acquisition and Literature Review • Strategy sessions • Comprehensive Literature Review • Survey of State DOTs Making Work Zones Work Better

  6. Classical Traffic flow Theory Making Work Zones Work Better

  7. Capacity Measurement • Maximum rate of flow (HCM) • Mean queue discharge rate • Hourly rate of flow under congested conditions • Flow rate at which traffic changes from uncongested to queued conditions Making Work Zones Work Better

  8. Traffic Measurement Techniques • Traffic Volume/Headway • Video surveillance • Inductive loops with counters • Road tubes with counters • Inductive counters • Speed measurement • Road tubes • Radar or Laser Making Work Zones Work Better

  9. Factors Influencing Freeway Work Zone Capacity • Work zone configuration • Highway grade • Presence of freeway ramps • Traffic stream make-up • Weather conditions • Intensity/duration of construction activities • Lighting Making Work Zones Work Better

  10. Volume Thresholds Making Work Zones Work Better

  11. Instrumentation and Field Data Collection • Design the surveillance setup, acquire hardware, and implement design • Field test and make adjustments • Field test at an actual interstate work zone • Collect data at various rural and urban work zone sites Making Work Zones Work Better

  12. Wind Research Tower Making Work Zones Work Better

  13. Surveillance Setup • Tall tripods from SkyEye Corporation Making Work Zones Work Better

  14. Roadside Setup Making Work Zones Work Better

  15. Base of Tripod Making Work Zones Work Better

  16. Connecting Cables Making Work Zones Work Better

  17. Autoscope camera and pan/tilt unit Making Work Zones Work Better

  18. Raising the tripod Making Work Zones Work Better

  19. Tie-down and Ground Anchor Making Work Zones Work Better

  20. Raised Tripod Making Work Zones Work Better

  21. Tripod Setup on a Bridge Making Work Zones Work Better

  22. Great Perspective View Making Work Zones Work Better

  23. Peripherals Making Work Zones Work Better

  24. Surveillance Setup Summary • Tripods are of adequate (not optimal) height • Stable, even in windy conditions. • Very flexible • Enhanced safety • Little or no effect on traffic operations • Need about an hour to setup completely • Breakdown takes about 1/2 hour Making Work Zones Work Better

  25. Projects • Collected data at 22 locations • First data collection on 9/12/01 • Summary: • I-85: 13 Projects • I-26: 6 Projects • I-77: 1 Project • I-385: 2 Projects Making Work Zones Work Better

  26. Making Work Zones Work Better

  27. Making Work Zones Work Better

  28. Data Collection Findings • Night time data collection not ideal • Autoscope not as effective • Volumes generally low except I-85 • Difficult to obtain specific information on location of closure in advance • Setup and procedures adequate for providing data to meet project objectives Making Work Zones Work Better

  29. Data Analysis • Graph and analyze data • Develop predictive model as a function of known model parameters • Traffic and truck volumes • Length of lane closure • Lane widths • Shoulder characteristics • Roadway grades Making Work Zones Work Better

  30. Underlying Concepts k=q/s (1) where: k = density (veh/mi) q = flow (veh/hr) s = speed.(mph) Can also be expressed in terms of average vehicle spacing: k=5280/spacing (2) Making Work Zones Work Better

  31. An Example of Capacity • Given: • Speed limit for short term work zone projects in South Carolina is 45 mph. • Average spacing in saturated conditions for a speed of 45 mph is 150 feet per vehicle. • Using eq. 2, k = 5280/spacing = 35.2 veh/mi. • From eq. 1, q = ks = 35.1 * 45 = 1584 veh/hr. Making Work Zones Work Better

  32. More Examples From eq. 1, for flow to stay constant, the density of the road must increase in proportion to the decrease in speed. This indicates that at lower speeds, there is willingness for cars to travel at closer intervals. Making Work Zones Work Better

  33. Combining Data to Facilitate Analysis • No single project completely follows Greenshields’ generalized form • Necessary to combine data • Difficult to isolate the characteristics of individual projects • Underlying assumption that all projects that were combined were homogeneous Making Work Zones Work Better

  34. Project Characteristics • Commonality: • Rolling terrain except I-26 south of Columbia • 12-foot lane widths • Similar taper lengths • Differences • Type of maintenance activities • Work zone length • Number of lanes downstream Making Work Zones Work Better

  35. Speed vs Flow 2 to 1-lane Discrete 5 minute flows Making Work Zones Work Better

  36. Speed vs Flow 2 to 1-lane 12 consecutive 5 minute periods Making Work Zones Work Better

  37. Considering Heavy Vehicles • Research has shown that heavy vehicles have a significant effect on capacity • Must be considered on a case by case basis • Most common approach is applying passenger car equivalents (PCE) Making Work Zones Work Better

  38. Headway (seconds) Methodology for Determining PCE • Measured headways by analyzing video • Used software developed for project (Satflo2) to record time and vehicle type • Results are tabulated and graphed Making Work Zones Work Better

  39. Headway Frequencies by Vehicle Type Making Work Zones Work Better

  40. Passenger Car Equivalents Grouped by Speed Speed (MPH) <15 15 to <30 30 to <45 45 to < 60 > 60 Project RV Truck RV Truck RV Truck RV Truck RV Truck 13 0 0 1.52 1.68 1.28 1.66 1.4 1.75 1.11 1.79 14 1.32 1.89 1.62 2.06 1.39 2.14 1.66 1.92 1.52 1.85 16 0 0 1.23 2.42 1.5 2.14 1.44 2.14 1.26 2.06 17 1.2 2.09 1.33 2.22 1.5 2.48 1.67 2.75 0 0 18 1.37 2.04 1.42 2.03 1.6 1.95 1.41 2.02 1.44 2.12 20 1.68 2.21 1.59 2.2 1.98 2.18 1.28 2.02 1.7 2.62 21 0 0 0 1.27 1.95 1.98 1.58 1.85 1.22 1.91 7 0 0 0 0 1.58 2.01 0 0 0 0 10 0 0 1.16 2.13 1.16 1.89 1.06 0 0 0 11 0 0 0 1.83 0 0 1.46 1.87 1.15 2.2 5 0 0 0 0 1.4 1.86 0 0 0 0 All 1.414 1.948 1.414 2.085 1.497 1.986 1.511 1.940 1.375 1.991 Making Work Zones Work Better

  41. Type of Vehicle Sample Passenger Car Equivalent Passenger car 10293 1.00 RV 470 1.44 Heavy Truck 2019 1.93 Average PCEs Used in Analysis Making Work Zones Work Better

  42. Speed vs Flow 2 to 1-lane Discrete 5 minute (PCE) Making Work Zones Work Better

  43. Speed vs Flow 2 to 1-lane 12 consecutive 5 minute periods (PCE) Making Work Zones Work Better

  44. Modeling Methodology • Model Speed vs Density Relationship • From the resulting linear model, substitute k=q/s • Determine maximum Flow in PCE • Adjust for other factors • Formulate model for application to specific short-term work zones Making Work Zones Work Better

  45. Speed vs Density 2 to 1-lane Discrete 5 minute (PCE) Making Work Zones Work Better

  46. Speed vs Density 2 to 1-lane 12 consecutive 5 minute periods (PCE) Making Work Zones Work Better

  47. Modeling Speed vs Flow • s = -0.395 k + 52.54 (5-minute discrete data) • s = -0.493 k + 54.11 (5-minute consecutive data) • Substituting k=q/s gives: • q= -2.53 s2 + 133 s (5-minute discrete data) • q= -2.03 s2 + 110.7 s (5-minute consecutive data) • where: q = flow (pcphpl) • s = speed.(mph) Making Work Zones Work Better

  48. Speed vs Flow 2 to 1-lane Discrete 5 minute (PCE) Making Work Zones Work Better

  49. Speed vs Flow 2 to 1-lane 12 consecutive 5 minute periods (PCE) Making Work Zones Work Better

  50. Estimating Capacity First derivative of the s vs q model is slope of the parabola dq/ds = -5.06 s + 133 5-minute discrete data dq/ds= -4.06 s + 110.7 5-minute consecutive data Slope of the parabola at maximum flow = 0. Setting the above equations = 0 give the speeds at maximum flow. Substituting into previous slide gives max flow (capacity): 1748 pcphl (5-minute discrete data) 1483 pcphl (5-minute consecutive data) Making Work Zones Work Better

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