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Appendix A: Detailed Traffic Analysis for I-270 Corridor

The following graphics and text summarize the rationale for the selection of September 13, 2007 as the "benchmark" non-congested day for I-270.

Paired Volume – Speed for "Summer-lite" Days

Scatter chart depicting volume-speed relationship for I-270 Northbound on July 30, 2007
Figure A-1: Volume-Speed Relationship for 7-30-07

  • Shows generally tight clusters of points – one cluster for each detector
  • 5-minute volumes go from about 100 to 475 veh
  • Each cluster has mostly high speeds of 60-75 mph; some speeds drop to 40 to 60 mph range
  • The points are consistent with HCQSM findings
  • Although this was a lightly congested day, the absence of data for MD 118 makes this day less useful for comparison to congested days

Scatter chart depicting volume-speed relationship for I-270 Northbound on August 6, 2007
Figure A-2: Volume-Speed Relationship for 8-6-07

  • Also shows generally tight but more dispersed clusters of points – one cluster for each detector.
  • 5-minute volumes go from about 100 to 550 veh.
  • Each cluster has mostly high speeds of 50-75 mph; some speeds drop to 30 to 50 mph range.
  • The points are consistent with HCQSM findings.
  • Presence of data for MD 118 makes this day more useful, but minor congested conditions at three detectors makes this day less useful.

Volume-Speed for a Summer and a "Special" Day

Scatter chart depicting volume-speed relationship for I-270 Northbound on August 13, 2007
Figure A-3: Volume-Speed Relationship for 8-13-07

  • Specific clusters of points are more dispersed
  • 5-minute volumes go from about 150 to 550 veh
  • Most speeds of 50-75 mph, but speed drops at:
    • MD121: many speeds of 20 to 50 mph
    • Comus Rd: some speeds of 30 to 50 mph
  • The points are consistent with HCQSM findings
  • Presence of congested conditions makes this day less useful for comparison purposes

Scatter chart depicting volume-speed relationship for I-270 Northbound on September 13, 2007
Figure A-4: Volume-Speed Relationship for 9-13-07

  • Specific clusters of points are fairly tight.
  • 5-minute volumes go from about 200 to 575 veh.
  • Each cluster has mostly high speeds of 60-70 mph; some speeds 50 to 60; few over 70; only two 5-minutes intervals just under 50 mph
  • The points are consistent with HCQSM findings.
  • Although not in "August", combination of high volumes, light congestion, and systemic demand reductions makes this day good for comparison.

Comparison of an Uncongested to a Congested Day

Scatter chart depicting volume-speed relationship for I-270 Northbound on September 13, 2007
Figure A-5


Scatter chart depicting volume-speed relationship for I-270 Northbound on October 11, 2007
Figure A-6

  • The 10-11-07 graph directly above, for a typical congested day shows:
    • Volumes vary from 225 to 575 vehicles (MD 118 had peak volume of 583 veh)
    • Five of the six detectors have speeds below 50 mph
    • At MD 121 many speeds less than 30 mph (slowest average speed of 17 mph)

Again, the points for both graphs are consistent with HCQSM findings.

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