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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2018. Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook. Washington, DC: The National Academies Press. doi: 10.17226/25007.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2018. Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook. Washington, DC: The National Academies Press. doi: 10.17226/25007.
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Page 2
Page 3
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2018. Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook. Washington, DC: The National Academies Press. doi: 10.17226/25007.
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Page 3
Page 4
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2018. Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook. Washington, DC: The National Academies Press. doi: 10.17226/25007.
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Page 4

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

1 Work zones (WZs) are necessary to periodically maintain, rehabilitate, enhance, and recon- struct this nation’s roadway network. The number of work zones that occur across the United States is significant. Over the course of 1 year, it was estimated that 26.5% of the National High- way System (NHS) experiences at least 1 day with a work zone in place on it (1). In the peak summer months, it was also estimated that 7.9% of the NHS had a work zone in place on it on any given workday. Unfortunately, evidence suggests that work zones tend to have an adverse effect on roadway safety. In 2013, a total of 527 fatal crashes occurred in work zones, leading to 579 fatalities (2). Overall, it was estimated that there were 67,523 crashes in work zones in 2013, of which 27.3% involved injuries or fatalities. These numbers also appear to be on the rise. In 2015, 642 fatal crashes occurred in work zones that resulted in 700 fatalities; both these figures are 20% higher than in 2013 (3). How Do Work Zones Affect Safety? In most situations, work must be performed at the same time that traffic is using the roadway. Depending on the work activities required, work zones can adversely affect the safety of traffic approaching and passing through them in several ways: • Work zones often require workers, equipment, materials, barriers, and traffic control devices to be in close proximity to travel lanes (see Figure 1). This reduces the amount of space available for vehicles to maneuver and recover from small mistakes in vehicle control and guidance actions. In other words, crashes occur more frequently in a work zone because there are more objects in closer proximity to traffic than existed prior to the creation of the work zone. • Work zones sometimes require the driver to make a temporary travel path change through a work zone when lanes are closed or shifted laterally (see Figure 2). Even if advance warning signs, channelization, and pavement markings are present to warn the driver, an increase in crashes can occur if the motorist is distracted from the driving task and fails to properly recognize the travel path changes. Crashes can also increase if the desired travel path itself is not clearly understood by all motorists. • As Figure 3 depicts, lane closures and other restrictions in the work zone reduce the traffic- carrying capacity of the roadway. Depending on the traffic volumes, these changes can create traffic slowdowns or stoppages that are not expected by approaching motorists. These violations of driver expectancy can lead to collisions between vehicles as well as impacts with objects as drivers swerve to avoid running into the back of another vehicle. C H A P T E R 1 Introduction

2 Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook Figure 3. Work zones often create slowdowns and congestion by reducing the traffic-carrying capacity of the roadway. Figure 1. Work zones often have objects in close proximity to open travel lanes. Figure 2. Work zones often require drivers to make temporary travel path changes.

Introduction 3 How to Estimate the Expected Effects of Work Zones on Crashes It would be beneficial for work zone designers and others to be able to predict the safety consequences of their proposed work zone design and management decisions prior to imple- menting them in the field. Examples of ways in which work zone crash estimates could be used include the following: • Quantify the safety-related benefits of completing the work faster, which could be used to help establish accelerated contract incentives that are in line with those expected safety benefits. • Estimate the expected impacts of safety countermeasures contemplated for use in the work zone as part of the transportation management plan (TMP). • Predict the differences in safety effects of alternative work zone design options, for exam- ple, the safety consequences of narrowed lanes or closed shoulders, reductions in available entrance ramp acceleration lane lengths, and so forth. Depending on the decision to be made, methods are needed to estimate the overall number of crashes expected to occur in a work zone and the incremental change in crashes resulting from different work zone features. Options do exist for estimating the overall work zone crashes to be expected. These options vary in terms of the amount of data and level of effort required, as well as in the level of accuracy to be achieved. Planning-level work zone statistical crash models have been developed to provide rough approximations of the number of crashes expected at a particular work zone. Mean- while, crash modification factors (CMFs) can be employed to estimate the incremental change in crashes due to alternative work zone features being considered, similar to procedures found in the Highway Safety Manual (HSM) (4). A CMF is a multiplicative factor used to indicate how a particular condition or feature increases or decreases the number of crashes expected from a set of base conditions. A CMF of 1.0 indicates that the feature has no incremental effect on crash risk. A CMF less than 1.0 indicates that the feature reduces crash risk, and a CMF above 1.0 indicates that the feature increases crash risk, relative to a set of base conditions. When multiple features are present, several CMFs are multiplied together to arrive at the overall estimate of the expected change in crashes. Although many CMFs have been developed for many different permanent roadway features in recent years, only a few work-zone-specific CMFs are currently available. In the absence of CMFs developed specifically for work zone features, the only available option is to utilize those that exist for permanent roadway features. In these cases, engineering judgment must then be applied when interpreting the results of the analysis. Purpose and Organization of this Guidebook This guidebook has been prepared to assist practitioners who develop phasing and staging plans for temporary traffic control through work zones to better evaluate the expected safety impacts of their plan decisions. The guidebook will also be useful to agencies, consultants, and contractors in their efforts to prepare and implement TMPs by providing a method for evaluating the potential benefits of certain safety countermeasures. Chapter 2 describes planning-level work zone crash estimation approaches available to prac- titioners. Planning-level methods can provide a rough approximation of either (a) the total number of crashes expected to occur overall during a particular work zone or (b) the additional number of crashes expected to occur above that normally occurring on the roadway when the work zone is not present.

4 Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook These approximations can be used in a number of ways—for example, estimating the safety benefits that would be achieved by accelerating work zone completion times by a certain amount or in determining an “expected” work zone crash rate per month that could be compared with actual crashes as a performance measure to see if the work zone is having a bigger safety impact than was expected. Chapter 3 describes how trade-off analyses of the expected safety impacts of alternative work zone design features, operational strategies, and countermeasures can be performed. A trade-off analysis utilizes available CMFs together with planning-level estimates of overall work zone crashes to estimate the incremental changes in crashes that would be expected due to a change in a particular feature or the inclusion of a particular countermeasure. Examples are provided at the end of Chapters 2 and 3 to illustrate how the methods can be applied to answer different types of “what if ” questions that a practitioner might encounter when developing a set of work zone plans or deciding whether or not to implement a particular work zone safety strategy. Chapter 4 provides a comprehensive catalog of CMFs that are available for potential use in evaluating the expected safety effects of work zone features and strategies. Each CMF is described and its potential applicability to work zone analyses critiqued; however, some features of interest do not yet have CMFs developed for them. For those features, a discussion is provided of the known operational effects.

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TRB's National Cooperative Highway Research Program (NCHRP) Research Report 869: Estimating the Safety Effects of Work Zone Characteristics and Countermeasures: A Guidebook provides practitioners who develop phasing and staging plans for temporary traffic control through work zones with guidance to evaluate the safety impacts of their plan decisions. There is limited data on work zone crashes and fatalities that address trends, causality, and the best use of resources to improve work zone safety. This guidebook provides clearer guidance to encourage the use of a data-driven, comprehensive, collaborative planning approaches for the selection and implementation of effective countermeasures to improve work zone safety.

Accompanying the report, NCHRP Web-Only Document 240: Analysis of Work Zone Crash Characteristics and Countermeasures documents the research results of multiple analyses focused on developing an improved understanding of work zone crash characteristics and countermeasure effectiveness.

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