Analytical Travel Forecasting Approaches for Project-Level Planning and Design (2014) / Chapter Skim
Currently Skimming:
Pages 1-55
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 1... ...
Background P A R T 1
|
From page 2... ...
3 The project-level traffic forecasting guidelines presented herein are intended to • Help standardize the traffic forecasting process for highway projects, • Give practical guidance to practitioners, • Give a high-level understanding to forecast users, and • Help define the current state of traffic forecasting practice. These guidelines are important since highway projects constitute a large portion of the U.S.
|
From page 3... ...
4NCHRP Project 08-83 follows the spirit and intent of NCHRP Report 255 in that it demonstrates how the large menu of forecasting practices that are currently available can be applied to produce reasonable forecasts for planning, design, and operations. Table 1-1 gives some perspective on how traffic forecasting has developed over time and where NCHRP Report 255 fits within this timeline.
|
From page 4... ...
Year Key Transportaon Organizaon (s) PlanningMilestones Technology Traffic Forecasting State of the Art Forecasng RelatedManuals 1930s Bureau of Public Roads (BPR 1915)
|
From page 5... ...
6Table 1-2. NCHRP Report 255 review summary.
|
From page 6... ...
7 There are 19 national sources; 20 state, county, and state/ MPO sources; and 3 other sources. The documents range from handbooks to guidelines to policy manuals to peer exchanges.
|
From page 7... ...
8factors and month-of-the-year factors, and vehicle classification considerations. Chapter 9: Traffic Forecasting Methods for Special Purpose Applications contains a host of topics: basic traffic forecasting deliverables, interpolation of traffic forecasts, vehicle mix accuracy, equivalent single axle loads, benefit/cost analysis, toll/ revenue forecasts, work zone congestion, environmental justice, and traffic impact studies.
|
From page 8... ...
9 C H A P T E R 2 2.1 Traffic Forecasting Data and Parameters Traffic data are the fundamental unit of information required for traffic forecasting. This includes average annual daily traffic (AADT)
|
From page 9... ...
10 capacity design decisions because traffic volume varies by hour and from day to day throughout the year. The formula for calculating the DHV is =DHV K AADT× Design hour factor (K)
|
From page 10... ...
11 Such inventory databases include historical traffic counts, intersection control, roadway classification, accident history, previous traffic forecasts, and so forth. The state and local area reports are useful to reference guidelines, local parameters and traffic conditions, and spreadsheets for use in traffic forecasting analysis.
|
From page 11... ...
12 far from the executive summary most consumers will read. Or, the work of several months will be crystallized into a few short presentation slides.
|
From page 12... ...
13 There are three categories of MOEs covered in this section: volume-based measures, travel-time-based measures, and accessibility measures. 2.3.1 Volume-Based Measures The definition of volume-based measures, their strengths and weaknesses, and their application and purpose are given in the following: • Definition -- Volume-based measures deal with the quantity of use of transportation facilities at a point on a transportation network, along several points on a screenline (an imaginary line that cuts across several roadways)
|
From page 13... ...
14 2.3.2 Travel-Time-Based Measures The definition of travel-time-based measures, their strengths and weaknesses, and their application and purpose are given in the following: • Definition. Travel-time measures describe the trip duration of travel by a vehicle between two points or the total effort incurred in travel by a group of travelers between or within regions.
|
From page 14... ...
15 This type of accessibility measure can be averaged over several zones comprising a region as: 1 1 A = A W W i ii = n ii = n ∑ ∑ Where W is the weighting factor, such as households or jobs (5) , i is the origin traffic analysis zone (TAZ)
|
From page 15... ...
16 • Transit assignment. Rather than give specific values for an impedance function, the report gives historical ranges of ratios of out-of-vehicle time values to in-vehicle time values.
|
From page 16... ...
17 guidance on the capacity and LOS analysis, and covers additional, related topics such as speed and volume data collection to quantify traffic demand and performance measures such as saturation flow and stopped delay. The HCM is a tool for estimating roadway and traffic performance and uses project-level traffic forecasts as an input.
|
From page 17... ...
18 Validation techniques are also available for trip generation, trip distribution, mode split, and assignment. Case studies provided in QRFM II include the Los Angeles freight forecast model, Portland metro truck model, Florida state freight model, and Texas state analysis model.
|
From page 18... ...
19 • Data requirements and preparation (e.g., OD trip table estimation from ground counts) , • Calibration targets and performance measures, • Error and model validity checking, and • Frequently encountered issues and suggestions for resolving them.
|
From page 19... ...
20 Toll Road Traffic & Revenue Forecasts: An Interpreter's Guide (18) provides practical information that could be used to help interpret toll and revenue projections.
|
From page 20... ...
21 detail in Appendix C The notable elements of the guidelines published by a selected number of states are described below: • Florida provides guidelines and techniques to forecast traffic and assess the impacts of land use on the transportation system.
|
From page 21... ...
22 C H A P T E R 3 This section presents an overview of traffic forecasting tools and methodologies. Of course, the most frequently used (and perhaps the best)
|
From page 22... ...
Table 3-1. Traffic forecasting tool/methodology topics.
|
From page 23... ...
24 3.2 State of the Practice of Travel Forecasting Models The state of travel forecasting has been described in many textbooks and reports, so this discussion is intended only to establish a baseline that can be referenced later when describing supplementary techniques or enhancements. The reader is directed to NCHRP Report 716 (6)
|
From page 24... ...
25 are the ends of trips at the location where the trip purpose is satisfied (completed) : – A commonly accepted method of calculating trip attractions is a set of linear equations of levels of zonal activity.
|
From page 25... ...
26 require that the user supply an initial (or "seed") OD trip table, as well as traffic counts.
|
From page 26... ...
27 • Person trips from a mode other than the automobile mode (e.g., transit) may be assigned to a modal network.
|
From page 27... ...
28 each origin zone and each destination zone that start at a particular time. Dynamic OD tables require dynamic traffic counts and a dynamic seed OD table.
|
From page 28... ...
29 The main hindrance to building a commodity-based model in urban regions is the absence of commodity flow data for zones smaller than counties. 3.2.2.6 Intersection Delay in Traffic Assignment Means for calculating intersection delay in travel forecasting have been rapidly evolving.
|
From page 29... ...
30 clean vehicle penetration in the marketplace, policies aimed at influencing vehicle purchase decisions of consumers can be analyzed using vehicle fleet composition models. (There is a distinction between fleet mix in the context of modeling for air quality analysis and truck percentages used for an actual forecast.
|
From page 30... ...
31 While induced travel may result exclusively from changes in network level of service measures due to a project-level improvement, it is also important to consider the longer term effects arising from land use changes triggered by enhancements in network accessibility (38)
|
From page 31... ...
32 applied to model traffic assignments. For each approach leg and the intersection as a whole, an aggregate node delay can be computed as the volume-weighted average of control delays for approach links to the node.
|
From page 32... ...
33 3.2.2.15 Interchange Ramp Terminals Interchange ramp terminals represent a special case of at-grade (service) interchanges of freeways with surface streets.
|
From page 33... ...
34 ramp metering is to achieve a smoother flow (and faster travel speeds) along the freeway.
|
From page 34... ...
35 3.2.4.1 Integrated Land Use -- Transport Models Integrated land use -- transport models are used to represent the interactions between land use changes and travel demand (43, 44)
|
From page 35... ...
36 Each of the above methods has certain advantages and disadvantages, and caution is to be exercised in deploying TOD choice models, particularly in the context of transit modeling, as transit service tends to vary by the time of day. Currently, most TOD models split trips after trip distribution.
|
From page 36... ...
37 In contrast, tour-based models -- although also incorporating heuristics and rules to some extent -- tend to be deeply nested logit model systems with a series of log-sum terms feeding up the chain to account for interdependency across choice processes. 3.2.4.6 Dynamic Traffic Assignment DTA refers to a class of mesoscopic traffic simulation methods in which travel demand between origins and destinations is routed through a network in a time-dependent way, and movements of individual vehicles are simulated to capture various traffic phenomena, such as delays, queues, and bottlenecks.
|
From page 37... ...
38 grams or systems. The jobs-created-per-dollar investment figure sometimes heard around discussions of proposed programs or infrastructure improvements are often derived from the types of economic impact analyses discussed here.
|
From page 38... ...
39 ing, it is useful to understand the types of travelers that use the roadway system in sufficient detail to assign values that correspond to the economic use of travel. Travel markets can include personal travel, commuter travel, on-the-clock automobile travel (travel on the way to conduct business)
|
From page 39... ...
40 3.2.4.10 Land Use Modeling Travel demand in a region or along a corridor is strongly influenced by the changes in land development patterns that occur over time. A host of land use variables serve as inputs for travel demand models, and the accuracy of project-level traffic forecasts is inextricably tied to the accuracy with which land use variables are predicted into the future.
|
From page 40... ...
41 3.2.5 Microscopic Traffic Simulation Some forecasts require information about the detailed interactions of traffic in small time increments over short distances (such as at driveways and intersections) and so require the use of tools more detailed than travel demand models.
|
From page 41... ...
42 has been coded correctly. Analysts' expectations also should be field verified.
|
From page 42... ...
43 3.3 State of the Practice of Data Inputs for Travel Forecasting Models Four-step travel demand models make use of a variety of input data sources. These data inform the model development process and establish the descriptive characteristics of the region being modeled.
|
From page 43... ...
44 Determinisc Methods (e.g., HCM) Vehicle Trajectory Based Methods (e.g., Microsimulaon)
|
From page 44... ...
45 category with an additional 3+ autos category. Larger numbers of categories are used if the analysis of household travel surveys supports greater segmentation in automobile ownership.
|
From page 45... ...
46 The ACS is a continuous survey conducted on a sample of the American population and supplements data collected during the decennial census. Questions asked by the survey touch on subjects such as housing costs and rent, utilities, race, gender, and other topics.
|
From page 46... ...
47 Links typically carry information about distance, travel time, and roadway capacities, and provide curvature to the network, while nodes establish intersections or opportunities to change travel direction in the network. For traditional models, link networks describe network connectivity movements along a series of interconnected nodes, for example, from Node 101 to Node 102 to Node 103.
|
From page 47... ...
48 packages all now have features built into them that allow the user to import GIS or LRS data and translate them into a format suitable for travel demand modeling. At the very least, geographically accurate distances can be retained in the model network.
|
From page 48... ...
49 3.3.5 Origin-Destination Studies OD data are used to develop trip distribution models, external trip tables, and baseline data for corridor studies. OD studies are a common feature of traffic and revenue studies conducted as part of a toll road financing study.
|
From page 49... ...
50 division of AM peak, PM peak, and off-peak or a four-part division of AM peak, mid-day, PM peak, and overnight are the most common TOD schemes encountered. Typical peak periods cover 3 hours each with the remaining periods making up the difference of the 24-hour day.
|
From page 50... ...
51 • Fuel consumption. Fuel consumption is typically a component of economic impact assessments and air quality analysis.
|
From page 51... ...
52 useful information for an agency's maintenance, rehabilitation, and reconstruction programs. 3.5 Defaults versus Locally Specific Parameters Ideally, travel forecasting models should be statistically calibrated to conditions for the area being analyzed.
|
From page 52... ...
53 • OD matrix growth factoring, • Time-series models, • Traffic impact study tools, and • Elasticity methods (see Section 10.3 of this report)
|
From page 53... ...
54 ing movements are known. The non-directional volume procedure requires more judgment on the part of the analyst as typically the turns are derived from knowledge of the non-directional approach link volumes and an estimate of the total turn percentage at the intersection.
|
From page 54... ...
55 The basic relationship is described by mathematical relationships between the dependent variable (either daily or peakhour trip ends) and independent variables (such as gross leasable square footage and number of employees)
|
From page 55... ...
56 categories of reviews are local government plan reviews and development of regional impact (DRI) reviews.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.