Guidebook for Developing Subnational Commodity Flow Data (2013) / Chapter Skim
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From page 59... ...
59 Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 3.1 Introduction This section provides a comprehensive examination of methods for developing subnational commodity flow data using roadside truck intercept surveys. Roadside truck intercept surveys are conducted by interviewing truck drivers while they are in the process of operating their vehicle.
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From page 60... ...
60 Guidebook for Developing Subnational Commodity Flow Data Many of these steps are interrelated, but the discussion of each step is ordered as shown in the above bulleted list. The description of each step is structured to focus on the following four key elements described in the Playbook (Chapter 6.0)
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 61 Implementation Process Because the major constraint in identifying roadside surveys is finding feasible locations, the first step in this process is to identify all of the feasible locations in the transportation agency's area of concern. This identification process will likely include truck weigh stations and truck inspection stations, but it also can include truck pull-out areas, public rest areas, and large parking lots located immediately adjacent to the corridor of interest.
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From page 62... ...
62 Guidebook for Developing Subnational Commodity Flow Data and often require security issues to be addressed prior to approval. Marine ports can be operated by state or local agencies or authorities, but many terminals are privately operated, with the public entity acting as the "landlord." In these cases, it is important to obtain approval from both the terminal owner and operator.
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 63 As noted earlier, it is oftentimes possible to conduct roadside truck surveys at rest areas, truck fuel stops, and designated truck parking areas near metropolitan areas. One of the disadvantages of using truck stops and truck parking areas is that trucks are not required by law to enter these areas, whereas they are required to enter weigh stations.
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64 Guidebook for Developing Subnational Commodity Flow Data Figure 3.2. Roadside survey sites for the SFTA origin-destination roadside survey in Washington.
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 65 Step 2 -- Questionnaire Design Key Considerations Designing roadside truck survey questionnaires requires a clear definition of the purpose of the data collection effort and also involves anticipating truck driver response to questions and a knowledge of the surveyors' familiarity with trucking operations. Additionally, there are tradeoffs that involve the number of questions asked relative to the response rate and the number of questions asked relative to the number of surveys that will be conducted over a fixed period of time.
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From page 66... ...
66 Guidebook for Developing Subnational Commodity Flow Data • Facility type (home, intermodal facility, warehouse, port, etc.) at last stop, next stop, last pickup/delivery, next pickup/delivery, ultimate origin and destination • Commodity being carried • Weight of cargo • Empty, partially loaded, or fully loaded • Identification of routes used or to be used on this trip An important distinction is between a location where a truck stops, a location where a truck picks up or delivers goods, and the locations where the cargo begins or terminates the shipment.
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 67 General information about the trucking company or broad questions about the trucking services offered include the following: • Location of truck's home base • Type of trucking firm (truckload, less-than-truckload, private) • Number of vehicles in fleet • Number of times that the truck driver passes the survey location (or alternative location)
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From page 68... ...
68 Guidebook for Developing Subnational Commodity Flow Data • Empty weight of the vehicle/trailer • Loaded weight (current payload weight) • Commodity on-board • Cargo origin (city, state)
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 69 Figure 3.3. Roadside survey questionnaire used in Washington state (EWITS)
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70 Guidebook for Developing Subnational Commodity Flow Data Figure 3.3. (Continued)
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 71 Implementation Process In practice, the selection of interview dates and times is typically constrained by the resources available for the survey effort, restrictions inherent at survey locations, and the number of locations that will be surveyed. Therefore, it is important to first understand whether there are limitations at any of the survey locations of interest.
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72 Guidebook for Developing Subnational Commodity Flow Data The survey schedule also was designed to minimize double counting of trucks on a single day. Sites surveyed on a given day were selected to be somewhat mutually exclusive.
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 73 Implementation Process Steps 1 and 3 have covered identifying a set of potential survey sites along with the dates and times of potential surveys at each site. This step addresses how many trucks to survey at each site, given the fiscal constraints of the roadside intercept survey process, and the types of freight questions that are usually answered through truck roadside intercept surveys.
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From page 74... ...
74 Guidebook for Developing Subnational Commodity Flow Data With the sample size determined for key variables, the next step is to determine the survey periods to consider as part of the analysis. Survey periods are generally determined based on observed fluctuations of truck count data at a particular location, but could vary based on a number of different factors that are known at a location.
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 75 If the maximum number of questionnaires collected at a given site is 20 percent of the total freight traffic, it is most likely to be the same on the additional days that the survey is conducted (assuming the traffic conditions are similar)
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76 Guidebook for Developing Subnational Commodity Flow Data Step 5 -- Interview Team Recruiting and Training Key Considerations The structure of the interview team will impact both the quality of the data and the cost of collecting the data. Using an interview team with significant experience in conducting roadside truck origin/destination surveys tends to provide the best data quality.
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From page 77... ...
Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 77 Regardless of the structure of the team, each data collection location should have a site supervisor that manages the coordination needed with facility operators, monitors the quality of the collected data, ensures that safety guidance is being adhered to at each location, and collects completed survey forms or data from electronic devices. This site supervisor should be an experienced transportation professional that has participated in previous roadside truck survey efforts.
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78 Guidebook for Developing Subnational Commodity Flow Data Using service club members was ideal in several respects. First, they proved to be a relatively large and geographically diverse short-term labor force, which matched the needs of the survey.
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 79 Step 6 -- Site Preparation and Traffic Control Key Considerations Site preparation and traffic control needs vary for each survey site. Coordination with the facility operator will be needed to ensure adherence to site-specific safety guidelines.
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80 Guidebook for Developing Subnational Commodity Flow Data be a survey team ahead was first placed at the entry of each weigh station to alert approaching drivers of the survey team's presence. Then, depending upon the individual officer(s)
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 81 The person who is first interacting with the driver has the added responsibility of maintaining traffic flow, asking drivers if there is an available place to park, and finding an available staff person to conduct the survey. It is important not to interfere with the normal operations of state patrol personnel.
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From page 82... ...
82 Guidebook for Developing Subnational Commodity Flow Data Implementation Process It is recommended that uniformed law enforcement officers and/or their vehicles are used at every roadside survey site location. This will happen automatically if weigh stations are used as survey site locations because uniformed law enforcement officers are always stationed at these facilities.
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From page 83... ...
Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 83 Implementation Process Quality control procedures need to be interspersed throughout the survey effort. One of the initial steps in quality control is for the survey site manager to review the survey process for each surveyor while they are in the field.
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84 Guidebook for Developing Subnational Commodity Flow Data The completed questionnaires were then thoroughly reviewed prior to being input into a relational database. During this process, much of the missing or incorrect data was corrected.
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From page 85... ...
Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 85 The first step is to divide the day into time periods that are believed to be consistent with fluctuations in truck activity. An hourly truck count curve at a location is a good indicator of these fluctuations.
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86 Guidebook for Developing Subnational Commodity Flow Data collected in different seasons to develop annual averages. This example is provided using an example from the EWITS survey using payload data.
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 87 age of through truck trips for each time period is shown in the fifth column of Table 3.2. The total expanded estimate of through truck trips is 40 percent.
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88 Guidebook for Developing Subnational Commodity Flow Data that each route in the database has been geocoded to represent all highway arcs comprising each observation, it becomes easy to provide this powerful analytical capability that develops a connection between all data attributes collected in the roadside questionnaire and the state geography or highway network. As an example, Figure 3.7 depicts the entire database of information (all routes)
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Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 89 Origins / Destinations Aberdeen, WA Addy, WA Adna, WA Alderdale, WA Algona, WA Amanda Park, WA Anacortes, WA .
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90 Guidebook for Developing Subnational Commodity Flow Data area on the network. This same type of analysis also may be performed for any attribute, including (but certainly not limited to)
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From page 91... ...
Collecting Subnational Commodity Flow Data Using Roadside Truck Intercept Surveys 91 3.3 Next Steps This chapter provides a detailed description of several elements involved in developing a roadside truck intercept survey. Nine basic steps have been identified that can be followed to fully implement a survey.
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