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Traffic Monitoring: A Guidebook

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5. Successful Traffic Monitoring

Study Plan

A successful traffic monitoring program includes a plan which outlines and provides details of the objectives that determine a successful outcome. A good plan will help to keep costs low and improve the quality of the collected traffic information that supports the decision-making process. For example, if the traffic volume is expected to be low on a roadway, then a manual count performed periodically may be a simple solution as opposed to the cost, installation, and maintenance associated with an automatic counter. A good study plan should include an inventory of existing data, a field site visit to potential count locations, and a defined set of goals. Detailing this information will assist in determining personnel and equipment needs and help define data analysis and data storage requirements in a cost-effective manner.

Existing Inventory

An agency can avoid unnecessary expenditure of money and time through a greater understanding of the traffic data that is currently available. Improved communication and coordination within an agency and with other agencies can help minimize the amount of data that needs to be collected. Other agencies, especially state governmental agencies, collect large amounts of traffic data throughout their respective jurisdiction. Data collected on nearby state highways could be used to assist in determining patterns that may exist near potential traffic monitoring sites or to develop adjustment factors that could be used to approximate traffic conditions near potential sites.

Field Site Visit

A potential site for traffic monitoring requires an on-site field visit to verify existing conditions and determine any available infrastructure. The field site visit is used to determine if a potential site is suitable for collecting traffic information, the type of data collection to be performed (manual or automatic), and all related characteristics needed to acquire good traffic data. For instance, an unobstructed view of the roadway is an important characteristic for conducting successful manual counts. For automatic counts, it is important to determine the traffic mix (including the number and type of trucks), geometrics of the roadway (number of lanes, lane widths, curvature of roadway, etc.), the pavement condition (is rutting evident) which could affect the operation of a road tube, and the power and data communication capabilities.

Determination of Goals

A traffic monitoring program requires a set of goals that will help accomplish the desired outcome. For one agency, that goal may be to determine the total traffic on multiple roads year to year, while another agency may be more interested in determining the type of vehicles that are using one particular section of roadway. Each agency needs to determine the type and amount of data to collect that will best support the decisions to be made to ensure a successful outcome. Upon determining the type and amount of data to be collected, an agency can then decide the best methodology to collect the data and the types of equipment that will be needed to accomplish this task, which should include the consideration of personnel, cost, and budget issues.

Quality Control/Quality Assurance

The objective of a traffic monitoring program is to provide good quality traffic information to support the future decisions of an agency. The information obtained from the traffic monitoring program is only as good as the quality assurance of the personnel, the equipment, and the traffic count data. The documenting of methodology and procedures of the various components of the traffic monitoring program can provide an avenue for facilitating training, reviewing performance, and identifying opportunities that can lead to corrective action when necessary.

Personnel

Traffic monitoring personnel represent a critical component of a successful program. Personnel will require training to effectively perform their duties. The training should include the following:

  • Proper procedures for conducting traffic counts;
  • Procedures for placing, retrieving, and maintaining automatic counters;
  • Proper procedures for monitoring performance standards of equipment;
  • Proper procedures for data analysis, sample estimations, and factoring; and
  • Proper procedures related to safety considerations.

Properly educating personnel will lead to a more motivated and dedicated work team generating consistent quality in data and greater safety awareness of staff resources. Reference materials are available to assist in this training and some have been listed in the back of this document.

Equipment

Traffic monitoring equipment is another critical component of a successful program. All equipment must be properly functioning and personnel must have a good working knowledge of the equipment to ensure valid data. Performance standards should be established to address factors such as equipment tolerances and failure rates and equipment down time durations to assist in determining availability and scheduling of equipment. Testing schedules should be established to ensure all equipment is operating properly. This can be performed by conducting a manual count simultaneously during an automatic count and comparing the results.

A general rule-of-thumb to follow is that the equipment is working properly if a comparison manual count is:

  • Within ± 10 percent of a portable (roadtube) automatic count;
  • Within ± 2 percent of a permanent (inductive loop sensor) automatic count;
  • Within ± 10 percent of a vehicle classifier automatic count; and
  • Within ± 15 percent of a weigh-in-motion GVW automatic count.

Quality assurance can be achieved through the establishment of procedures which document all activities related to a traffic monitoring program.

Quality assurance can be achieved through the establishment of procedures which document all activities related to a traffic monitoring program.

Count Data

Count data that is collected from the traffic monitoring program must be checked for validity to ensure quality control has been achieved. Corrupted data should not be included in the database. Corrupted data can be caused by an equipment malfunction due to a power failure, a sensor malfunction, or a communication error. These malfunctions will typically show up in the data as a series of "0" recordings or as a series of extremely high values for vehicle volume locations. In weigh-in motion counts, invalid data recordings may be due to vehicles not passing over the sensor properly. This data should not be included in any summarization reports, but should be retained for training and audit purposes.

Data collected during holidays, sporting events, parades, or traffic incidents are referred to as "atypical" (10). This type of data should be analyzed with scrutiny and may need to be excluded from any summarization reports, unless the specific purpose was to monitor this type of event. Extended periods of inclement weather are also considered "atypical" and are usually excluded from analysis, especially for short term counts.

Data Analysis, Summarization, & Reporting

Data analysis and summarization of the data collected from traffic counts is another critical part of a traffic monitoring program. General principles have been established to ensure that traffic data is analyzed and summarized in a consistent manner from one agency to the next. Data analysis involves the examination of the traffic data to determine any variability in traffic that can be used to identify travel patterns or trends over time. Understanding and identifying the variability in traffic will assist in determining, applying, and evaluating adjustment factors for traffic counts.

Variability in Traffic

The variability in traffic is inherently derived from a large transportation system that enables a diverse multitude of users to travel at various times and days. That is, traffic can vary from day-to-day, week-to-week, and season-to-season. Additionally, traffic can vary by location, such as comparing traffic data from different road types or comparing traffic data from different regions within a state.

This variability in traffic must be taken into account when trying to compare traffic counts from one location to another because different types of counts and equipment may have been used at different locations. For instance, one site may use a permanent counter which counts traffic everyday of the year. This site could be used to examine the seasonal variability and trends that may exist at that location. This information could then be used to develop seasonal adjustment factors that can be applied to other locations within the area that use shortduration counts.

In another example, a count location may use a portable counter that collects classification data over several weeks. This site could be used to examine the variability in vehicle type that may exist at that location. This information could then be used to develop axle correction factors that can be applied at other count locations within the area to estimate traffic volumes more accurately.

Factoring Counts

Adjustment factors are needed to modify traffic counts in order to account for the variability in traffic. Adjustment factors can be calculated for various uses including adjusting seasonal variations in traffic, adjusting axle counts to vehicle volumes, and providing annual growth estimates.

It should be noted that care must be taken when using adjustment factors to ensure the proper application of the factors in order to reach an accurate conversion. Most State highway agencies have established traffic monitoring programs and have the experience to assist in correctly applying adjustment factors.

Initially, "factor groups" are developed within a traffic monitoring program to determine how continuous and shortduration counts can be compared to other similar locations. One method is done by grouping roadway types by their respective functional classification. This allows for the adjustment factors from a "sample" roadway that behaves or operates in a particular manner to be applied to similar roadway types in the same region. Another method is referred to as a cluster analysis. This method determines which count stations are most similar based on a statistical algorithm. It may be that neither of these methods will work as a "factor group" for a particular area. In that case, the agency will have to determine an appropriate "factor group" plan.

Short-duration traffic counts are conducted in order to help reduce costs of traffic monitoring programs by extending the use of staff and equipment to multiple locations for shorter periods of time. Consequently, adjustment factors are needed to adjust short-duration traffic counts, which may include only a few days of data, into an annual estimate of daily traffic. This is typically performed by utilizing appropriate adjustment factors derived from nearby continuous count locations, such as ATR locations, which have a similar "factor group." The most common approach to converting a short-duration count to an annual average daily traffic (AADT) estimate utilizes the following equation:

AADThi = VOLhi * Mh * Dh * Ai * Gh
where
AADThi = the annual average daily travel at location i of factor group h
VOlhi = the 24 hour axle volume at location i of factor group h
Mh = the applicable seasonal (monthly) factor for factor group h
Dh = the applicable day of-week factor for factor group h (if needed)
Ai = the applicable axle correction factor for location i (if needed)
Gh = the applicable growth factor for factor group h (if needed)

Source: Traffic Monitoring Guide, FHWA, May 2001.

Conversely, the same equation can be used to compute a particular adjustment factor (i.e., seasonal, day-of-week, axle correction or growth) from a continuous count location. To calculate this, the observed AADT from an ATR is divided by the various factors (if applicable) to obtain the desired adjustment factor to be used for that particular "factor group."

Factoring Methods within Parks

Park and recreational roadways typically have different travel variability and travel patterns than rural or urban roadways. Therefore, it may not be appropriate to arbitrarily apply the same methodology as used on highways, to collect and adjust short-duration counts on park and recreational roadways.

Extending Count Period

Many park facilities only collect 48-hour short-duration counts, typically due to budget constraints and/or personnel limitations. While this methodology may apply well for urban commuter routes where travel patterns are more predictable, it does not effectively capture the variability of recreational traffic in and around parks.

It is suggested that to accurately capture day-of-week variations in recreational traffic, the traffic monitoring program should conduct short-duration counts for a period longer than 48 continuous hours (preferably one-week) made in different months.

While this practice may include a longer and more costly short term budget, research has shown that within a given park "the traffic patterns observed during each day-of-week and month-of-year combination were not statistically different from one year to the next" (16). This implies that shortduration counts could be adjusted using historical AADT data, if that data is more statistically valid, and therefore, fewer counts might be conducted from year to year.

Interagency Cooperation

In some instances there may not be a continuous counter available, for example within a park facility, that can be used to acquire adjustment factors which could then be applied to short-duration count locations within the park.

It may be appropriate to utilize nearby continuous counters from another agency (i.e. State, County, City, etc.) to obtain adjustment factors that could be applied to the short-duration counts within the park.

Through inter-agency cooperation, costs to collect the count information could be reduced by sharing the effort between different agencies. It has been shown that roadways located within close proximity (less than 20 miles) of parks have similar variability characteristics (16). Once again, the "factor group" must be considered when utilizing adjustment factors that are obtained from outside "similar" locations. Recreational traffic in and around parks will have different variability when compared to the traffic on rural or urban roads not within the immediate vicinity of the park.

Ratio of Annual Average Daily Traffic (AADT) to
          Monthly Average Daily Traffic (MADT). Monthly Adjustment Factors at Yosemite National Park Ratio of Annual Average Daily Traffic (AADT) to Monthly Average Day Of The Week (MADT) Traffic. Day-of-Week Adjustment Factors for July at Yosemite National Park
Reporting Data

Reporting of traffic count data is performed after it has been collected, analyzed, and summarized. Quality traffic related decisions can be made and improved through the effective reporting of the traffic data. Presentation of the data can take many forms including standard reports, tables, and graphical representations. The reports and tables might include summaries of vehicle volume, vehicle classification, or weight measurements as relevant to decision maker needs.

Graphical presentation of data will help to show the representation of adjustment factors more clearly. Seasonal factors being shown from month to month will show the differences in travel patterns related to seasonal influences while day-of-week factors will show the variations of travel on a daily basis (see example figures).

Data Storage

Retention and storage of traffic data is another important aspect of a successful traffic monitoring program. Data retention guidelines are generally based on length requirements set forth by the agency that is performing the traffic monitoring program and are based on the following (9):

  • legal requirements of a governmental (county, state, federal, etc.) agency;
  • required data retention periods of other related data;
  • utilization of the data;
  • availability of computer resources;
  • life expectancy, space requirements, and cost of data storage resources; and
  • available budget for the current and future years.

At a minimum, data to retain should include all original or "raw" traffic data collected as well as dates and times of efforts, all adjustment factors, and all adjusted traffic data including data that has been analyzed and summarized into reports. This information may be accessed in the future to determine trend patterns over time. Additionally, all methodology and guidelines related to the traffic monitoring program should be retained for auditing and training purposes.

Over time a traffic monitoring program will collect a large amount of data that needs to be stored (or warehoused) on a medium which can be easily accessed at a later date. Data storage can include such common current storage devices as a computer server drive, a portable hard drive, or digital video disk (DVD). These generally low cost storage devices allow for dependable long-term storage of data while providing the functionality needed to access the data for analysis. In some instances, various companies offer a secure web site for the storage of large amounts of data. The medium chosen to store the data should be updated to new formats that conform to the modernization of technology as needed. It is suggested that off-site storage of duplicate data can provide additional protection from fire or other actions deemed hazardous.

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