Section 1

Work Zone and Traffic Characteristics


1.1   DEFINITIONS

Work Zone is defined in the Highway Capacity manual as an area of a highway in which maintenance and construction operations are taking place that impinge on the number of lanes available to traffic or affect the operational characteristics of traffic flowing through the area.

Road User Costs in the work zone are added vehicle operating costs, delay costs, and crash costs to highway users resulting from construction, maintenance, or rehabilitation activity.

1.2   WORK ZONE CHARACTERISTICS

In order to calculate work related road user costs the characteristics of the work zone must be defined. Work zone characteristics of concern include such factors as work zone length, number and capacity of lanes open, duration of lane closures, timing (hours of the day and days of the week) of lane closures, posted speed, and the availability and traffic characteristics of alternative routes. The strategy for the maintenance of traffic should include any anticipated restrictions on the contractor's or maintenance force's hours of operations or ability to establish lane closures.

Each work zone established over the analysis period can have different impacts on traffic flow and the associated user costs. Whenever characteristics of the work zone must be defined and evaluated as a separate event.

The duration of a work zone (i.e. the overall length of time a facility or portion of a facility is out of servace) can range from sporadic daily lane closures for maintencance to serveral months for bridge deck replacements.

1.3   TRAFFIC DIVERSION

Traffic demand is generally determined based on the need to use the facility. Traffic volume during work zone operations may or may not be the same. Some portion of the traffic normally wanting to use the facility may divert to other routes when work zones are established.

Vehicles use a given facility because it offers, what the vehicle operators perceive to be, the least expensive combination of vehicle operating and time delay costs, consistent with safety requirements. When faced with restricted flow, or even the anticipation of restricted flow, vehicle operators who normally use a facility will exercise one of several options. The potential vehicle operator responses are categorized as follows:

Hang Toughers - This group continues to use the facility as they always have. They are primarily users with little, or no, option. They (1) must make the trip, (2) they must make it at a specific time, and (3) either don't know of, or don't have, alternative routes or modes to choose from. These users pay the full price of the work zone and have little effect on other facilities in the corridor. In rural areas the predominate choice of through traffic will be to tough it out, as these users generally must make the trip and do not have available alternative routes unless formal detours are established.

Time Shifters - Time Shifters have the "luxury" to travel on the facility or other route at a different time - generally a time well outside of the restricted flow period. These users lessen their impact by sharing the impact with other vehicles by "invading" their time slot. These users also have little effect on other facilities in the corridor, but do impact hourly traffic distribution.

Detourees - Detourees either seek out and use alternate routes, or are forced to negotiate detours established by the highway agency. These operators also lessen their impact by sharing the impact with other vehicles by "invading" their routes. They tend to trade off anticipated time delay for additional travel distances and associated vehicle operating costs. In urban areas this could include users who switch modes. Detourees can have significant impact on overall road user costs of alternative routes.

Trip Swappers - Trip swappers have the "luxury" of totally abandoning the trip or seeking other destinations when the cost, in terms of time and money, becomes too great. Historically, this group consists primarily of shopping and social/recreational trip makers. While their behavior may diminish the road user costs impact in the work zone they adversely impact businesses along the route in question. More recent trends in people working out of the home and Telecommuting may have a significant effect on work trips in the future.

In simple cases, where either work zone disruption is tolerable or alternative routes are limited, the estimated Average Daily Traffic (ADT) during the duration of the work zone can be anticipated to continue on the facility and the work zone analysis can be limited to the existing facility.

In more complex situations where existing traffic would face intolerable work zone disruptions, it is entirely possible that total travel demand and hourly distribution on the facility may change when the work zone is established. When demand changes, the road user cost analysis may have to expand beyond the existing facility and include road user costs on major alternative routes. When preliminary analyses of travel demand show that work zone related road user costs are unreasonably high, allowable work hours may need to be restricted, early contract completion incentives may be appropriate, or an alternative design may need to be considered.

1.4   TRAFFIC CHARACTERISTICS

Road user costs are directly dependent on the volume and operating characteristics of the traffic on the facility. Each construction, maintenance, and rehabilitation activity generally involves some temporary impact on traffic using the facility. The impact can vary from insignificant for minor work zone restrictions on low volume facilities to highly significant for major lane closures on high volume facilities.

The major traffic characteristics of interest for each work zone include such factors as the overall projected Average Daily Traffic (ADT) volumes, the associated 24-hour hourly traffic distributions, and the vehicle classification distribution within the traffic stream. Each of the major traffic characteristics is discussed in the sections that follow.

1.5   AVERAGE DAILY TRAFFIC (ADT)

Current weekday and weekend ADT volumes can be obtained from the traffic monitoring section. When using projected ADT volumes, it is generally assumed traffic patterns remain the same. However, consideration should be given as to whether traffic using the facility will continue to use the facility when work zones are established and traffic flow is restricted.

1.6   HOURLY TRAFFIC DISTRIBUTION (BY DIRECTION)

The 24-hour hourly traffic distribution during work zone operations is essential to be able to compare the unrestricted demand on the facility with the facility's ability to carry that traffic through the work zone. On all routes, distinctions between weekday and weekend traffic hourly distributions are important. Further, when work zones are proposed on recreational routes during seasonal peak periods, seasonal ADT traffic distribution is important.

If current 24-hour hourly traffic distributions are not available default values may be used. The New Jersey Department of Transportation has developed hourly traffic percentages for various functional classes of roadway for each County. These values are based on existing 24-hour weekday traffic volumes and are provided in Appendix B (zip 711k). It should be noted that the use of these percentages is most appropriate for preliminary analyses.

1.7   VEHICLE CLASSIFICATIONS

Road user costs are a composite of the costs of all affected highway users. Highway users are not a homogeneous group. They include commercial and non-commercial vehicles ranging from motorcycles and passenger cars through the heaviest trucks. Appendix A of the FHWA Traffic Monitoring Guide, Third Edition (February 1995) includes 13 different vehicle classifications. These different vehicle types have different operating characteristics and associated operating costs. Further, the value of time differs between vehicle classes. As a result, road user costs need to be analyzed for each major vehicle class present in the traffic stream.


Last Document Correction:
March 9, 2006