Work Zone Mobility and Safety Program

Work Zone Performance Measurement Using Probe Data

1. Introduction

1.1 Background

Performance measures establish quantitative measures for assessing an organization's performance. Establishing and estimating performance measures is an important first step in improving organizational performance – performance measures provide the basis for quantitatively comparing current and past performance to help determine if process changes are improving performance. In fact, the act of establishing performance measures helps an organization focus on improving performance by encouraging the organization's staff to focus on improving those measures.

In September 2004, the Federal Highway Administration (FHWA) updated the work zone regulations at 23 CFR 630 Subpart J to encourage the collection and use of work zone safety and mobility data, stating that:

  • States shall use field observations, available work zone crash data, and operational information to manage work zone impacts for specific projects during implementation.
  • States shall continually pursue improvement of work zone safety and mobility by analyzing work zone crash and operational data from multiple projects to improve State processes and procedures.

In other words, the rule implies that agencies should use data to generate performance measures at the project and process levels in order to improve both the safety and mobility of work zones. FHWA's guidance document on implementing this rule1 makes this implication more concrete by recommending the application of performance measures in four areas of interest: safety, mobility, construction efficiency and effectiveness, and public perception and satisfaction.

Work zones present challenges when it comes to collecting data for mobility performance measures since, by definition, work zones are not fixed in space or time. Traditional fixed sensors are often not located where needed to support work zone performance measures and sensors that are located in a work zone may be out of service because of work zone activities. Because a work zone is temporary, it may not be cost-effective to deploy traditional fixed sensors specifically to support work zone performance measures. Even at a large, long-term work zone where it could be cost-effective to deploy traditional fixed sensors, the location of work within the work zone may vary from day to day, making it difficult to identify appropriate locations at which to deploy sensors. Even when these challenges are overcome, there can be biases in the resulting performance measures because data is more likely available from work zones in urban areas (where fixed sensors are more likely to be available) and larger work zones (in which the larger potential impacts make the collection of traffic data more likely).

Recent advances in probe data technologies provide the potential to overcome many of these challenges. Some probe vehicle technologies (e.g., Bluetooth detectors) are inexpensive to deploy and easy to move, allowing an agency to cost-effectively monitor work zone traffic at specific work zones. Several commercial vendors of mobile traffic data now provide nationwide traffic data coverage based on probe data sources. This provides the potential to use a single, uniform source of data that can be used to support performance measures across all work zones in a State.

Several organizations have already begun to take advantage of this potential. Some have deployed probe based data collection systems to monitor work zone traffic. Others have developed work zone performance measurement systems that leverage probe data available from commercial vendors. The purpose of this report is to help other organizations take advantage of the potential for probe data to support work zone performance measurement programs by (a) providing guidance for using probe data for work zone performance measures and (b) describing the experiences and lessons learned from organizations that have begun doing so.

1.2 Work Zone Traffic Impacts and Mobility Performance Measures

Generally, the mobility problems associated with a significant work zone project include:

  • Increase in congestion due to reduced speed limits, narrowed lanes, and lane closures.
  • Increase in congestion caused by work zone related crashes, which can result from increased friction in traffic flow (e.g., at lane closure merge areas) and unexpected queues resulting from work zones.
  • Reduced access to businesses and/or special events.

Safety problems associated with work zone projects include worker safety, decrease in road safety, and, where applicable, decrease in pedestrian safety.

Several agencies have been successful in using safety and mobility performance measures to assess their performance in these areas, to identify deficiencies or gaps in their approach to project delivery, and to make improvements. Some performance measures are used programmatically to assess the extent to which policies, processes, and procedures are working to limit safety and mobility impacts of work zones. A common example is the number of work zone crashes and worker accidents used to assess work zone safety. Other performance measures are used at a project level to assess the performance of an individual project and to provide the opportunity to implement project-specific mitigation steps if expected performance levels are not being met. For example, a mobility measure such as average travel time through a work zone could be monitored and mitigation steps taken if the work zone travel time exceeds a pre-defined threshold.

Probe data is a potential source of performance data for work zone mobility performance measures.

Because probe data is most useful for assessing mobility measures, this report focuses on mobility measures. The literature further divides the data needs for work zone mobility performance measures into three categories: (1) performance data, (2) exposure data, and (3) indicator or stratification data. This is the specific focus of this report – the use of probe data as a source of performance data for work zone mobility performance measures.

The literature also describes a number of specific performance measures for measuring work zone mobility:

  • Delay per vehicle
  • Queue length
  • Duration of queue
  • Volume to capacity ratio
  • Level of service
  • Volume/throughput
  • Percent of time operating at free-flow speeds
  • Percent of work zones meeting expectations for traffic flow
  • User complaints

Probe data can be useful for assessing most of these performance measures. (Section 2 of this report provides detailed guidance on using probe data to assess mobility performance measures.)

1.3 Introduction to Probe Data for Work Zone Mobility Performance Measures

In general, probe data is defined as data that is generated and collected from moving vehicles, with the objective of measuring certain performance characteristics at the location of individual vehicles. Traditional approaches to traffic data collection involve the use of roadside infrastructure to collect traffic data (e.g., speed, volume, occupancy) from passing vehicles. In some cases – typically to support research rather than traffic operations – this data was supplemented by probe data in which vehicles driven by researchers drove through the study area and recorded time-stamped information about the vehicle's position.

In the past few decades, the evolution of telecommunications and wireless technologies has opened up a world of opportunity to collect traffic data in alternative ways. These technologies support probe-based systems that rely on using increasingly ubiquitous Bluetooth devices to identify vehicles, Global Positioning System (GPS) technologies, or cellular location systems. These offer broader coverage, with the potential to cover major arterials.

Mobile data exists in different forms, with different contents, and can be collected in different ways. Mobile data can be categorized into three major classes, based on the data collection methods used:

  1. Floating car data collected by electronic transponders. In this method, electronic transponders (tags) are placed on vehicles and electronic devices for reading those tags are placed along the roadway to determine when each vehicle passes those locations. The Automotive Vehicle Identification (AVI) technique is one such example which is covered widely in the literature [1]. Other examples include electronic toll data collected at toll booths, Bluetooth data collected by roadside Bluetooth receivers, etc.

Figure 1. An Example of a Roadside Bluetooth Detector
A roadside bluetooth detector mounted at the rear of a post-mounted roadway sign.
Source: Photo provided by BlueToad.

  1. GPS based mobile data. In this case, probe vehicles are equipped with GPS receivers that determine vehicle position from signals received from earth-orbiting satellites. The positional information determined from the GPS signals is transmitted to a control center to display real-time position of the probe vehicles. Usually GPS mobile data mainly come from certain types of vehicles, particularly fleet management services (e.g., taxis and trucks). The Connected Vehicle program conducted by U.S. DOT is now offering a new opportunity for collecting GPS based mobile data.
  2. Cell phone based mobile data. With this type of system, every switched-on mobile phone becomes a traffic probe in the network. The location of the mobile phone is usually determined by means of triangulation or by the hand-over data stored by the network operator, so cell phone system can provide time stamped information on vehicle locations. This information can be translated into road segment travel times. In contrast to the first two categories, cell phone based mobile data requires no hardware in cars and no specific infrastructure needs to be built along the road.

Overall, mobile data differ dramatically in nature from traditional fixed sensor data. The advantages and some shortcomings of mobile data are summarized in Table 1.

Table 1. Pros and Cons of Mobile Data for Work Zone Performance Measures
Advantages Disadvantages
  • Low or no cost in installation and maintenance
  • Wide geographic coverage (freeways and arterials)
  • Finer resolution (individual vehicle and shorter measurement time interval)
  • Includes travel time information
  • Not affected by traffic interruptions or bad weather conditions
  • Can provide details on roads without fixed sensors currently in place
  • Can provide accurate real time information to reflect shifts in routes and origins and destinations
  • Provides ability to assess impacts of operational changes before and after work zone actions
  • Less experience with analyzing data
  • Technology is not as mature as fixed sensors
  • No occupancy or traffic density information

These unique properties of mobile data make it practical to be incorporated into traffic analysis and help improve the accuracy and relevance of work zone monitoring.

These differences in the nature of mobile data have also resulted in differences in how traffic data is obtained. Traditionally, each agency was responsible for obtaining traffic data along the roadways they managed, deploying and maintaining sensors where needed in order to do so. Several companies are now taking advantage of probe data to provide traffic data as a commercial service. These services often provide capabilities that differ from those of traditional traffic monitoring.

Probe data, then, provides two basic approaches for improving work zone mobility performance measures:

  1. Inexpensive roadside detectors (e.g., Bluetooth) can be deployed to collect traffic data for specific work zones. The primary benefits of this approach are that the detectors are relatively inexpensive and easy to relocated and produce travel time data, which is difficult to produce with traditional traffic monitoring techniques.
  2. Traffic data from commercial vendors can be used across all work zones within the geographic coverage of the vendor. The primary benefit of this approach is more uniform coverage across all work zones and availability of more detailed data both within the work zone and on nearby roads. The cost can also be low, particularly for States that are already contracting for probe data from a commercial vendor.

But, there are also disadvantages and limitations. The data available is often limited to travel times and the technologies and analysis techniques involved are not as mature as with fixed sensors. These advantages and disadvantages will be examined in more detail in the remainder of this report.

1.4 Document Overview

This report consists of three major sections:

  1. This section introduces the reader to the topics of work zone performance measures and the use of probe data to support those performance measures.
  2. The second section provides a more detailed description of probe data and guidance on how to use probe data for work zone performance measures.
  3. The third section includes case studies of several recent applications of probe data for work zone performance measures.

The report includes two appendices. The first describes the future of probe data and the second summarizes results of a simulation study on the use of Bluetooth to estimate queue length.

1 Implementing the Rule on Work Zone Safety and Mobility (23 CFR 630 Subpart J), FHWA-HOP-05-065. Available at http://ops.fhwa.dot.gov/wz/rule_guide/. (Verified 7/25/2013.) [ Return to note 1. ]

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