INTRODUCTION

Work zones can cause significant traffic congestion and safety impacts. Agencies strive to manage these impacts as best possible so as to meet the needs of its customers, complete the project effectively, and support regional mobility and economy. On September 9, 2004, the Federal Highway Administration (FHWA) amended its regulation (23 CFR Part 630 Subpart J) that governs traffic safety and mobility in highway and street work zones (1) . One provision in the rule requires states to use both traffic safety and operational data to help improve agency processes and procedures to address work zone impacts. Specifically, states are encouraged to develop and implement systematic procedures that assess work zone impacts in project development, and to manage traffic safety and mobility impacts during project implementation. One way that agencies can improve these efforts is through the use of performance measures.

WHAT ARE WORK ZONE TRAFFIC SAFETY AND MOBILITY PERFORMANCE MEASURES?

Work zone performance measures are metrics that help to quantify how work zones impact travelers, residents, businesses and workers. Some measures describe the impacts of a specific work zone (project-level metrics), whereas other measures describe the impacts of a set of work zones (agency program-level metrics). Some examples of possible work zone performance measures include:

• The average peak-period delay to vehicles entering a work zone,
• The percent of time that a work zone queue exceeds an agency-established allowable threshold,
• The average percent increase in crashes in freeway work zones in a given state, and
• The number of injury crashes occurring during temporary lane closures on particular types of roadways.

WHY SHOULD WORK ZONE PERFORMANCE MEASURES BE USED?

Safety or mobility impacts predicted for a work zone during planning and design can be quite different from what is ultimately experienced at a site. Every work zone is unique, and the combined effects of design decisions, work phasing and sequencing operations, and impact mitigation strategies implemented at a site can be challenging to predict beforehand. Work zone performance measures help agencies understand how their decisions affect work zone safety and mobility. This allows agencies to improve conditions at current work zones as well improve how they make decisions for future work zones.

Work zone traffic safety and mobility performance measures have a variety of other uses to state departments of transportation (DOTs) and other highway agencies. Specifically, performance measures allow agencies to:

• Assess whether they are meeting their own goals and objectives regarding acceptable work zone traffic impacts;
• Identify specific problems or issues that may be occurring at a particular work zone;
• Review and improve their work zone policies and procedures;
• Review and improve the accuracy of traffic impact assessment tools being used during work zone planning and design;
• Improve the prediction of expected benefits of impact mitigation strategies that may be included in the transportation management plan (TMP) of a project; and
• Better "tell their story" about work zone impacts, and effects of mitigation strategies implemented to reduce those impacts, to elected officials and to the public.

Work zone performance measurement need not be an overwhelming activity for an agency. Rather, agencies should choose a few good measures carefully based on their particular needs and characteristics, communicate those to all stakeholders and staff, and track them clearly, seriously, and consistently.

HOW CAN WORK ZONE PERFORMANCE BE MEASURED?

Not all work zones need to be measured, and those work zones that are measured do not necessarily need to be monitored at all times. Work zone performance measurement should be driven by agency and other stakeholder needs and priorities. One advantage of establishing work zone performance measures is that it focuses attention on what is considered important to the agency and stakeholders.

To be useful, it is important is for agency staff to understand:

• What is being measured,
• How it is being measured, and
• Why it is being measured.

Ultimately, an agency may begin work zone performance measurement efforts on a limited basis by focusing on a few key performance measures for only a few projects each year. Then, as agency staff becomes more familiar with how and why work zone performance measures are used, the number and types of measures examined and the projects included in the analysis can be expanded.

Three basic types of performance measures are useful to quantify work zone safety and mobility impacts:

• Exposure measures,
• Safety measures, and
• Mobility (traffic operations) measures.

Exposure measures describe the amount of time, work activity periods, roadway space, and/or vehicle travel that a work zone or a collection of work zones affects or requires. Exposure measures are also usually needed as a denominator for safety or mobility performance measures (such as the number of crashes per million-vehicle-miles of travel through a work zone). Both output and outcome exposure measures of performance can be useful. Output-based measures describe the amount of effort or other resources being expended (such as the number of hours of temporary lane closures a contractor has implemented at a project), whereas outcome-based measures describe how many vehicles or vehicle-miles of travel occur through a work zone or group of work zones.

As the name implies, safety measures describe how crash risk has changed for the individual motorist and/or for the traveling public in general, relative to pre-work zone levels. Safety measures can also be defined for contractor or agency personnel working on the roadway. Most safety measures are outcome based, such as the increase in injury crashes or the change in overall vehicle crash costs in a work zone. However, a few agencies do track output-based measures of certain efforts to reduce crashes as surrogates to safety impacts, with the assumption that such efforts result in improved safety. An example of such a measure would be the hours of additional law enforcement assigned to work zones in an agency’s jurisdiction.

Finally, mobility (traffic operations) measures describe how travel mobility has been affected for motorists (and potentially other types of travelers as well). Travel delays caused by work zones are an obvious example of this type of performance measure. However, other measures related to, or correlated with, delay are also available. For example, queue length and duration is a performance measure used by some agencies. Other agencies use average speed through the work zone as a measure. Most traffic operations measures are outcome-based. However, output measures of efforts to reduce delays and queues might also be important to an agency.

Often, the safety and mobility data are combined with exposure data into other performance measures, such as the number of traffic crashes that occur per million vehicle miles of travel in a work zone. This process allows agencies to compare safety and mobility impacts across projects and over time, and also to aggregate those impacts across multiple projects. In other cases, additional computations and analyses may be required to obtain useful performance measures. An example of such analyses is when data is collected on the length and duration of queue lengths at a work zone, and the agency is interested in knowing how these queues translate into vehicle delays through the work zone.

OBJECTIVE OF THIS PRIMER

This primer has been developed to assist agencies in establishing and monitoring a useful set of work zone safety and mobility performance measures. The primer includes guidance on both project-level and agency program-level performance measure needs, defines a number of possible work zone performance measures that agencies can use, and describes the methods and technologies that are available to gather data to monitor them. The primer also outlines procedures on how to calculate specific performance measures from different types of work zone traffic monitoring data, and on use of the measures across multiple projects to assess compliance to agency policies and goals.

Much of the information presented in this primer was based on pilot study research of work zone performance measures for projects in several states nationally. Multiple methods of collecting traffic operations data and computing performance measures from the data were used simultaneously at each project in the study. This simultaneous analysis allowed a comparison of the relative level of effort and precision of performance measures obtained from each data collection method. The reader is encouraged to review the final report of that pilot study effort for additional insights into work zone traffic safety and mobility performance measurement (2) . In addition, this primer contains information from a recent scan of state DOT best practices for work zone impact assessment, data collection, and performance measurement (3) .

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