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This case study is about a Traffic Impact Assessment for a proposed site development in Clifton Park, New York Problems focus on the chapters of the HCM dealing with: • interrupted flow facilities (especially signalized intersections) • arterials • freeway interchanges • arterial weaving
After Working Through this Case Study You Should be able to: • Analyze the operation of signalized & unsignalized intersections, and urban arterials using the HCM. • Understand what input data are required and the assumptions that are commonly made regarding default values for the HCM procedures for these facilities. • Know the appropriate kinds of analysis that should be undertaken for existing facilities, including the scope of the analysis. • Understand the limitations of the HCM procedures and when it is appropriate to use other models or computational tools. • Know how to reasonably interpret the results from an HCM analysis and how these results can be used to support a particular decision regarding a change to a transportation system.
The table shows ways to classify traffic analysis problems that are appropriate for analysis with the HCM, cells with blue text will be discussed in this case study
Multi-lane arterial Serves trips to & from: 3 major shopping centers a local school district campus many commercial & residential areas What would be reasonable time periods to collect data? Characteristics of the Corridor - AM / PM peaks & peak of the traffic generator - Possibly Saturday midday & the Friday peak hours (e.g. for a shopping center)
For this case study the goal is to mitigate negative impacts resulting from traffic related to the site & general background traffic growth. • Related to the goals and objectives are performance measures, including: • Delay • Level of service (LOS) • Total vehicle hours of travel • Total vehicle miles • Air pollution outputs • Noise impacts What scenarios will we need to model to address these goals? What other potential issues may need to be addressed as part of this development?
What Analyses to Perform • It is necessary to examine each intersection in each time period • Also some specialized analyses (heavier-than-typical traffic conditions) • You might also want to do a system-level analysis to ensure that you have accounted for all the impacts that arise
What Analyses to Perform What other analyses might be necessary? For this TIA, how many intersection-level analyses will be involved? Study the freeway interchange at 7 locations (see HCMAG), equaling 84 more analyses 84 - 7 intersections: A through G - 3 time periods: AM peak, PM peak, Weekend peak - 4 conditions: existing, future without site, future with site, future with mitigation Instead of conducting all 168 analyses, we’re going to focus on specific problems that let us illustrate how to use the HCM
Tools to Use It is important to select analysis tools that strike a balance between the amount of effort they require & the amount of insight they provide 1 – Highway Capacity Manual: for all the intersection analyses and location-specific analyses at the freeway interchange 2 – Computerized arterial signal system optimization procedure: for arterial analysis of intersections A - D What are some tools that should be used for this case study and why?
Data What types of input data are required? • Facility-related information (e.g., number of lanes, lane widths, lane configurations) • Traffic-related information (e.g., vehicular & pedestrian volumes for all conditions) • Operational information (e.g., signal timings)
Problem 1: Maxwell Drive/Clifton Park Blvd Intersection (existing conditions and the with-site conditions analyses) • 1a: PM Existing • Base Case • Arrival Time Changes • Sensitivity to Data • Skipped Phases • 1b: PM With-Site Conditions Analysis • Configuration Issues • HCM Planning Method • Cycle Length Discussion • Critical Movement Techniques • Operational versus planning analyses • Uncertainty
Maxwell Drive Characteristics The Maxwell Drive / Route 146 intersection is signalized and fully actuated To the east ~2,000 ft east is the intersection of Clifton Country Rd / Route 146 To the west ~4,000 ft is the intersection of Moe Rd / Route 146 To the north ~300 ft is the intersection of Park Ave / Maxwell Dr. All three of these upstream intersections are signalized and fully actuated.
Maxwell Drive Base Case Phasing What other data do we need for this analysis? 20–40 sec 8–12 sec 10–18 sec (skipped ~ ½ time) The cycle length ranges from 30-70 seconds & averages 48 seconds
Maxwell Dr – Base Case Analysis Characteristics: • Peak Hour: 5 – 6pm • Intersecting vehicles = 2877 • PHF = 0.94 for all approaches • 7% trucks • Arrival Types • 2 for EB approach • 3 for WB and SB approaches
Observations? Maxwell Dr – Base Case Analysis Base case results for signal timings that equalize the delays for the critical movements in each phase What are the movement specific delays? What are the average queue lengths for each approach?
Observations? Arrival Type Changes What are the effects of changing the arrival type? If the coordination gets worse (arrival type 1), the EB left-turn delay could increase to 27.2 sec (45% more than the base case)
Observations? Data Sensitivity If data were collected for the same time period at the same location on multiple days, what kind of variance would you expect? It could be a lot or a little. In this case, are there any significant differences? No, the differences are minor How can you study the impacts of potential differences? With sensitivity analyses.
Skipped Phases What combinations are possible with this configuration? Since the HCM doesn’t ask for dual-ring phasing how should this phenomenon be modeled (min greens, max greens, gaps, etc.) Adjust the modeled signal timings so that they reflect an average cycle given that specific phase(s) will sometimes be skipped
Skipped Phases Do skipped phases make a difference? YES!!! Why is this delay 3 times greater for Dataset 7 than the base case? The cycle length is 5 sec. longer What happens if when the other phases are adjusted to reflect the change in the phase 2 timings? The WB thru delay is less than the base case, but all of the other movements have more delay
Observations? Sub-Problem 1b: Maxwell Drive PM Peak Hour – With-Site Conditions Configuration Issues What are some reasonable configurations for the new site? What are some of the tools to develop a signal timing plan? • HCM planning method for signalized intersections • Critical lane analysis
For the HCM Planning Method what needs to be supplied? Intersecting volumes Left-turn treatment (protected, permissive, compound, etc.) Number of lanes (left, through, and right) Peak hour factor Min & max cycle lengths Coordination situation (yes or no) If parking is present What does the model determine? Lets us know if the configuration will work Reports the capacity condition (above capacity, at, nearly at, below, etc.) Presents a phasing plan HCM Planning Method
Critical Movement Technique What are the differences between scenarios? Different lane use plans produce very different cycle lengths (also very different phasing plans) Combining certain movements, like the NB & SB rights and the EB & WB lefts, can get more productivity out of the intersection and reduce the cycle length
Observations? Critical Movement Technique Do you see any weaknesses in the critical movement analysis approach? How do shared lanes complicate the situation? What about intersections where there are no left-turn lanes?
Operational vs Planning Analyses What are the differences between scenarios? We can make the signal work, in an operational analysis, for the conditions that the planning analyses suggested should work. We have also seen that the cycle lengths are sometimes different.
Uncertainty Issues For this intersection we will look at: • Base Case (C-4) • Double Lefts, NB & SB (C-7) • Separate NB/SB phases (C-8) • 30% more site-generated traffic (+30%) • 30% less site generated traffic (-30%) Compare and contrast the different scenarios. Which seems to have the least delay?
Conclusions Current Configuration • We found that the intersection’s geometry will have to change substantially. • We needed a new NB approach and we’ve found it useful to reconfigure the SB approach. • Our best solution uses 3 lanes SB (left, through, and right) and 3 lanes NB (left, through, and right). • We’ve used the PM with-site condition to look at: • Changes in LOS due to the addition of the site-related traffic • The relationship between geometric improvements and LOS • Differences between planning and operational analyses • The role of uncertainty in affecting the results obtained Best Solution