3. IDENTIFYING THE WORK ZONE PERFORMANCE MEASURES OF INTEREST
The first step of the pilot test process was to identify a desired set of performance measures to target. Input was solicited from members of the AASHTO Subcommittee on Traffic Engineering (SCOTE) and the Subcommittee on Systems Operations and Management (SSOM) work zone technical teams. Field personnel comments obtained through the recent Texas DOT work zone performance monitoring study were also reviewed (4). These efforts identified three key points with regards to work zone safety and mobility performance measure identification:
- The measures selected must relate to the safety and mobility goals and objectives that the agency has established for itself. Examples of such goals and objectives established by agency policy or procedures include maximum tolerable queue lengths and duration, maximum motorist delays, target reductions in work zone crash rates, and minimum customer satisfaction ratings.
- The measures must adequately capture both the breadth and depth of motorist impacts so that the trade-offs between accommodating motorists’ needs and other requirements of a project (time, access, cost, etc.) can be adequately balanced. The effect that a project has upon motorists can vary dramatically from one project to the next. One project may result in a few work periods of intense motorist congestion and delays with several other work periods of no impacts to motorists, whereas another project may have a fairly small but consistent impact upon motorists throughout the duration of the project. When examining across all motorists passing through the work zone during the project, the average vehicle delay for those two projects may be quite similar. However, they would likely be perceived quite differently by the public. Focusing on measures that only examine one aspect of those impacts (e.g., maximum individual motorist delay) may not accurately reflect the overall picture of what happened, and lead to erroneous decisions as to how to best complete the work, or the mitigation strategies that may be needed. Rather, several measures are often needed.
- The measures must be sensitive to the alternative strategies available to agencies for accommodating traffic. Over the course of a project, certain tasks or phases may generate some impacts while others have little or no impact. When considering how to best accommodate travel during a work zone project, agencies have the option of choosing when work should occur, how much of the roadway to allocate to the work and how much to leave for traffic to use, and what (if any) techniques or strategies that may be used to enhance roadway capacity, improve safety and/or traffic flow, etc. Measures should be selected to allow agencies to assess whether such decisions did or did not work. It may be desirable to compute measures separately by project phase; by work activity condition (during work activity when lane closures are present, during work activity without lane closures, during periods of no work when lane closures are present, during periods of no work activity and no lane closures are present); and by various time periods during the day or night (peak-period, daytime off-peak, nighttime, weekend, etc.).
- Different audiences may need or desire different performance measures. For instance, measures useful to DOTs for work zone mobility impacts (e.g., percent of work zones meeting the agency queue threshold) may be different than those used to describe impacts to the public, local residents, or nearby businesses (e.g., average work zone delay).
Together, these issues indicate the importance of having a suite of performance measures that can be tailored as desired to the needs of a particular agency. The measures need to reflect project level as well as individual traveler level impacts, and the strategies used to accommodate travel. Methods of stratifying these impacts within project phases, periods of work activity, or when specific traffic-handling strategies are in effect are also needed. Ultimately, these measures may be aggregated across multiple projects for regional and statewide assessments. With these requirements in mind, measures were developed to address these needs under each of the following five priority categories:
- Exposure
- Traffic queuing
- Traveler delay
- Travel time reliability
- Safety
EXPOSURE MEASURES OF INTEREST
Exposure measures describe the amount of time, roadway space, and/or vehicle travel that a work zone (or a collection of work zones) affects or requires. Both output and outcome exposure measures of performance can be useful. For example, output measures of exposure are needed for tracking contractor activity and efficiency. The contractor typically has considerable leeway in determining when and how specific tasks are performed. Exposure measures that capture how much effort (work activity) is being expended, and when such work is being accomplished, relative to the total time available for doing the work, is a key indicator of the level of importance the contractor is giving to the project. Another dimension of exposure that relies on output-based measures is the roadway capacity restrictions required. The number of lanes closed (relative to the number of lanes normally open), the hours when the lane closures occur, and the lane-mile-hours of closure are additional ways to capture highway agency and contractor decisions on when and how work was accomplished. Whereas some work zone design features (crossovers, lane shifts) may decrease capacity slightly through the work zone, the magnitude of decrease will typically be much less than that experienced by a full lane closure. Table 1 presents a list of proposed performance measures pertaining to work zone exposure. Both output- and outcome-level measures are described, as is the rationale for including each one in the table.
| Measure | Measure Type | Definition | Use |
|---|---|---|---|
| % Calendar Days (or Nights) with Work Activity | Output |  |
Some projects are issued based on a total calendar day bid; for such projects, comparison of work effort to available calendar days is appropriate. |
| % Available Working Days (or Nights) with Activity | Output |  |
Other projects are issued with restrictions on which days or nights work can occur; therefore, comparison of work effort should be based on when work is allowed. |
| Average hours of work per day (or night) | Output |  |
The amount of time typically used by the contractor per shift can then be used to extrapolate total work hours over an entire project, or across similar projects. |
| %Work Activity Hours with: 1 lane closed, 2 lanes closed, 3 lanes closed, etc. |
Output |  |
The amount of time multiple lanes are closed can be compared against the amount of delay and queuing generated to evaluate the adequacy of lane closure policies. |
| Average Lane Closure Length | Output |  |
Average lane closure length can be used to extrapolate across and projects not being monitored as closely. |
| Lane-mile-hours of closures | Output |  |
Lane-mile-hours of closures can be useful for assessing contractor and work crew productivity. |
Vehicles passing through the work zone in evaluation period during:
|
Outcome | ∑ vehicles entering work zone during periods of (activity, lane closures, inactivity, etc.) | Vehicle exposure during various time periods is needed to estimate delays on a per-vehicle basis; in some instances, it may be desirable to assess safety impacts on a per-vehicle basis as well. |
Vehicle-miles-of travel in evaluation period during:
|
Outcome | ∑ vehicles entering work zone during periods of (activity, lane closures, inactivity, etc.) × length of (work zone, activity area, lane closure, etc.) | Vehicle-miles-traveled is a traditional denominator used to establish vehicle crash rates. |
Indicators of vehicular travel that passes through the work zone (both the number of vehicles and the corresponding vehicle-miles traveled) are outcome-level exposure measures. These measures allow mobility and safety impacts to be normalized on a per-vehicle or per-vehicle-mile basis, and can be further stratified by other output-level exposure measures listed above (i.e., vehicle-miles-traveled (VMT) during hours of work activity). These statistics are outcome measures (as opposed to output measures) because the creation of significant congestion and delays due to roadwork can significantly alter driver route choice diversion decisions, which affects the amount of traffic traveling through the work zone.
One of the challenges of utilizing VMT as an exposure term is in defining what length or limits should be used in the computations. For major roadway rehabilitation and reconstruction projects, temporary geometric changes over the limits of the project suggest the use of the project limits as the basis for estimating VMT exposure. For these types of projects, essentially all traffic and VMT would be considered affected by the work zone (travel and shoulder lane width restrictions and other geometric constraints would be present even when no work is occurring). However, for other projects, vehicle exposure during the times when temporary lane closures are occurring (during hours of a hot-mix asphalt overlay job, for example) may be the only exposure of interest. Here, the limits of the project may not be as relevant as the length of actual lane closures each day or night. Monitoring and computation of VMT exposure to this activity is straightforward; relating this value to the total VMT in the section of roadway would be less so. For these projects, estimating exposure in terms of the percentage of average daily traffic on the facility (or in terms of the total amount of traffic on the roadway during the project) would likely be more relevant in determining the percentage of the driving public that likely traveled through the work zone.
QUEUE MEASURES OF INTEREST
Both queuing and traffic delays reflect the effect of work zones on traveler mobility, and are obviously correlated with each other. However, safety considerations relative to the formulation of queues (i.e., increased risk of rear-end crashes, ensuring that sufficient advance warning signing is located upstream of the start of queuing, etc.) make direct monitoring of their length and duration important as well. Also, the fluctuations in traffic demands and other factors will affect queuing patterns that develop during each work shift, and across work shifts over the duration of a project. Measures that can assess how frequently specific levels of queuing are being exceeded (and by how much) are important indicators of these fluctuations. Table 2 presents a suite of queue measures of performance identified for this pilot test effort. As noted in the table, these measures can be defined relative to specific agency thresholds, such as the frequency of queues at a project that exceed a given length for a given duration.
| Measure | Measure Type | Definition | Use |
|---|---|---|---|
| % of work activity periods when queuing occurred | Outcome |  |
This measure can also be defined relative to a minimum threshold (i.e., the percent of work activity periods when queues exceeded a given length or duration). |
| Average duration when a queue was present | Outcome |  |
The average duration of queues can be useful to agencies in deciding how far upstream from the work zone to begin warning motorists about possible delays. |
| Average length when a queue was present | Outcome |  |
The average length of queues can be used to estimate average vehicle delays if travel time data are not being collected to directly measure delays. |
| Maximum length of queue during evaluation period | Outcome | Max (queue length measurements during evaluation period) | Maximum queue lengths are tracked to assess whether advance warning signing is being placed far enough upstream of the lane closure to adequately warn approaching motorists. |
| % of work activity when queue > 1 mile | Outcome |  |
The threshold distance (1 mile) can be changed to reflect agency policy objectives. Also, multiple measures using multiple thresholds (i.e., 1 mile, 2 miles, etc.) could be computed to give a more complete picture of queuing patterns occurring at a site. |
| Amount (or %) of traffic that encounters a queue | Outcome | ∑ traffic when a queue is present during time period of interest | Assessing the number of vehicles or percent of daily traffic that encounter a queue is useful for evaluating appropriate beginning and ending times of temporary lane closure periods. |
These measures are defined relative to a no-queue condition during the periods of work activity; that is, the assumption is that any queues that develop are the direct result of the work activity and temporary lane closures required. If queues were occurring during the same times before the project began, these measures would need to be defined in terms of changes from their pre-work zone levels in order to isolate the effects of the work zone from these normal congestion impacts. Most agencies strive to avoid closing lanes when congestion already exists at a location, reducing the need to factor in existing queues.
DELAY MEASURES OF INTEREST
From an agency perspective, delays are directly relevant in estimating road user costs caused by work activities, which in turn drive decisions regarding bidding approaches and contracting strategies employed, incentives and/or disincentive provisions of the contract, etc. From this perspective, total delays summed across all users are of most interest. Conversely, from a user satisfaction perspective, delays experienced by individual motorists encountering the work zone are better indicators of mobility impacts. Recognizing that individual delays can vary significantly over the course of a project or even hours of a particular work shift, multiple indicators may be needed to capture both the extreme and “typical” impacts are of interest. Another measure, percent of work activity time when motorist delays are exceeding some threshold will be useful to agencies that have identified a maximum tolerable level of motorist work zone delay. Table 3 presents the delay measures of performance targeted in this pilot test.
TRAVEL TIME RELIABILITY MEASURES OF INTEREST
Another dimension of assessing travel quality pertains to the reliability of trip travel times on a given roadway or route. Drivers want dependable travel times so that they can better plan their departure and arrive at a destination near a desired time (13). A given roadway may have an average travel time associated with it, but frequently have incidents or other events occur that temporarily increase the travel time. Roadways with highly variable travel times require that motorists “buffer” in more time in their departure time decision to ensure that they are likely to arrive on time, even though there is a chance that they will arrive much earlier than necessary if travel conditions are favorable. The prevailing approach to measuring travel time reliability is to compare the average travel time for roadway segment in a particular time period (i.e., peak period) to the 95th percentile travel time in that time period; the greater the difference between the average and the 95th percentile travel time, the more unreliable travel conditions on that roadway are considered to be. This difference is usually divided by the average travel time to normalize it as a percentage, and is then referred to as the buffer index:
| Measure | Measure Type | Definition | Use |
|---|---|---|---|
| Total delay during entire evaluation period | Outcome | ∑ Vehicle-hours delay in evaluation period | This measure can be multiplied by the value of time to estimate additional road user costs being caused by the project. |
| Total delay per work period | Outcome |  |
This measure is useful (when multiplied by the value of time) for estimating the user benefits achieved by accelerated construction techniques. |
| Total delay per work period when queues are present | Outcome |  |
This measure can be useful when compared to the total delay per work period to determine the variability in work zone mobility impacts that are occurring from work period to work period. |
| Average delay during work activities per entering vehicle | Outcome |  |
This measure can be useful when queues occur fairly frequently at a work zone, and can be compared fairly easily across projects. |
| Average delay during work activities per queued vehicle | Outcome |  |
This measure can be useful when queues only occur sporadically during a project, targeting the subset of vehicles that actually encounter a queue. |
| Maximum individual delay during evaluation period | Outcome | Max (individual delay per vehicle during evaluation period) | The upper bound on maximum individual delay experienced during the project can be helpful in responding to public complaints about perceived level of work zone mobility impacts. |
| % of vehicles experiencing delays greater than 10 minutes | Outcome |  |
This measure indicates the percentage of drivers experiencing greater than tolerable delays (10 minutes should be changed to reflect the agency’s acceptable delay threshold). |
The use of the 95th percentile travel time as the upper limit implies that someone who allows that amount of time for their trip would arrive late no more than once every 20 days (but would typically be early). Other upper limits could be used as well and interpreted similarly. For example, use of the 80th percentile travel time in the above computation would correspond to arriving late no more than once every five days (i.e., once in a typical work week).
Work zones can also temporarily reduce the capacity of the roadway, and influence the reliability of travel times on a roadway. This measure is of most interest where there is already some degree of travel time unreliability on the roadway segment, and a question exists as to whether (and how much) a work zone further affected reliability. In locations where work occurs on roadway segments and during times that are normally congestion-free, the relevance of a change in travel time reliability is less since the impacts of the work zone are fully characterized through the frequency and extent of delays and queues that develop. Consequently, a travel time reliability measure was only examined at two pilot test locations where traffic congestion and queuing was already occurring on a regular basis prior to the start of the construction project.
SAFETY MEASURES OF INTEREST
Safety measures of performance are needed to assess changes in crash risk relative to pre-work zone levels for both the individual motorist and for the driving public in total. Crash-based performance measures will not be very useful for work zones that are short in length or duration, or occur on lower-volume roadways, as the numbers of crashes themselves will be too small to draw solid conclusions. Although some agencies may propose to monitor operational indicators such as speeds or changes in speed in lieu of actual crash data, there does not yet exist credible research that correlates work zone speeds (or other operational measures) to safety impacts. Table 4 presents two proposed traffic safety measures of work zone performance. These changes could be stratified by time-of-day and/or work activity type if desired. Individual crash rates, expressed in terms of crashes per million-vehicle-miles traveled (MVMT) or changes in that crash rate when a work zone is present, capture the effect that the work zone had upon individual motorist risk. Conversely, the change in crash costs of a particular project or project phase represents the effect upon the driving public in total. A project done primarily at night may have a higher crash rate increase per MVMT, but may result in far fewer crashes in total than if the project had been done during daytime hours, due to the much lower traffic volumes present at night. Similarly, construction strategies implemented to reduce project or project phase duration could result in a higher crash rate per MVMT, but again lead to fewer total crashes if the project duration (and thus the amount of traffic passing through the work zone in total) was reduced significantly.
| Measure | Measure Type | Definition | Use |
|---|---|---|---|
% change in crash rate during work zone
|
Outcome |  |
Changes in crash rates per million-vehicle-miles-travelled reflect the additional risk per mile experienced by a motorist traveling through the work zone. |
| Change in crash costs from expected no-work zone crash costs | Outcome |  |
Differences in total crash costs combine both changes in frequency and severity due to the work zone together in one measure. |
Δ crash severity type iwz = change in number of crashes of a given severity in the work zone from what would have been expected over the same time period at that location if the work zone were not present
In addition to a convenient way to characterize the total effects of a project (or group of projects) on safety, the use of crash costs best captures the trade-offs that may exist in establishing policy decisions or safety mitigation procedures that influence crash severities more so than total crashes (certain strategies may increase certain types of less-severe crashes but decrease the more severe crashes that could occur). This approach was used in a recent comparison of nighttime and daytime work zone safety (14). If used, a decision must also be made as to how crash costs themselves will be estimated. Recent FHWA guidelines could be used as a starting point (15).
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