Before addressing road user cost calculation procedures, it is helpful to understand the Road User Cost Components. There are a total of ten potential work zone related road user cost components that can occur. Three components are associated with a "Base Case" situation where traffic operates under "Unrestricted Flow" conditions. Four components are associated with a "Queue" situation where traffic operates under "Forced Flow" (Level of Service "F") conditions. Two components are associated with a "Circuity" situation where traffic is forced to utilize a detour to avoid a highway work zone, and the final component is associated with "Crash Costs". Each of the potential road user cost components is discussed in the sections that follow.
Work zones restrict traffic flow either by restraining the capacity of the roadway or, as a minimum, by posting lower speed limits. Figure 2.1 shows unrestricted flow conditions at a work zone. All traffic that flows through the work zone, at a minimum, must slow down while traveling through it and then accelerate back to normal operating speed. This is commonly referred to as a speed change and results in three work zone related road user cost components. The cost components associated with the unrestricted flow case in Figure 2.1 are described below.
Speed Change VOC (Vehicle Operating Cost) - This is the additional vehicle operating cost associated with decelerating from the unrestricted upstream approach speed to the work zone speed and then accelerating back to the unrestricted approach speed from the work zone speed after traversing the work zone.
Speed Change Delay - This is the additional time necessary to decelerate from the unrestricted upstream approach speed to the work zone speed and then to accelerate back to the unrestricted approach speed after traversing the work zone.
Work Zone Delay - This is the additional time necessary to traverse the work zone at the lower posted speed. This depends on the normal and work zone speed differential and length of the work zone.
If traffic volume remains below work zone capacity, the added road user costs are limited to the above three components and the analysis is relatively simple. In most cases delay times remain relatively low and represent more of a minor irritation and inconvenience than a serious problem.
When traffic demand exceeds work zone capacity, traffic flow breaks down and a queue of vehicles develops as shown in Figure 2.2. It is important to note that the queue does not form in the work zone itself, but in the upstream approach to the work zone. Once a queue develops, all approaching vehicles must stop at the approach to the work zone and creep through the length of the physical queue under forced flow conditions at significantly reduced speeds. As long as the traffic volume exceeds the work zone capacity, the length of the queue grows. When the traffic volume eventually falls below the work zone capacity, vehicles then leave the queue faster than they arrive and the length of the queue shrinks and eventually dissipates over time. When capacity is reduced on high traffic facilities, it is not uncommon for queues to develop in the morning peak traffic period, dissipate, and then redevelop in the afternoon peak traffic period.
Queuing situations impose four more work zone related road user cost components that are a direct result of the queue. They are in addition to the "Unrestricted Flow" added road user costs and only apply to vehicles that encounter a physical queue. The cost components associated with the forced flow case in Figure 2.2 are described below.
Stopping VOC - This is additional vehicle operating cost associated with stopping and accelerating back up to work zone speed.
Stopping Delay - This is additional time necessary to come to a complete stop (instead of just slowing to the work zone speed) and then accelerating back to the work zone speed.
Queue Delay - This is additional time necessary to creep through the queue under forced flow conditions.
Queue Idling VOC - This is the additional vehicle operating costs associated with "stop and go" driving in the queue. The operating costs include fuel, engine oil, maintenance, and depreciation.
The conceptual analysis presented in Figure 2.1 and Figure 2.2 is geared primarily to freeway conditions. Conceptual analysis of facilities with at-grade intersections would also incur speed change, stopping, delay, and idling costs, but at a much higher frequency due to intersection control devices and turning movements.
Circuity is a term used to describe the additional mileage that users travel, either voluntarily or involuntarily, on a detour to avoid a highway work zone or queue situation. Circuity situations impose two more work zone related road user cost components that are a direct result of the detour. They can be in addition to or in lieu of the "Unrestricted Flow" and "Forced Flow" components. If traffic is forced to detour, the associated cost components are described below.
Circuity VOC - This is the additional vehicle operating cost associated with traveling the excess distance the detour imposes.
Circuity (Detour) Delay - This is the additional time necessary to travel the excess distance the detour imposes and this depends on the travel time and travel length differentials.
Crash Costs are a function of the crash rate for the work zone and for the facility in absence of work zones. Crash rates are typically based on the number of crashes per vehicle miles of travel. Crash rates are commonly specified as crashes per 100 million vehicle miles of travel (100 M VMT).
Overall crash rates for the various functional classes of roadway are fairly well established. Crash rates for work zones, however, are not. While there is a limited amount of work zone crash data, the validity of the data used to compute the crash rates is sometimes suspect. Crashes that occur in work zone generated queues are not always classified as "Work Zone" crashes. Probably even more importantly, it is difficult to accurately quantify the work zone exposure rate (i.e. the length of the work zone and the hours and days the work zone queues are in place). Further, the crash rate, while generally higher in work zones than non-work zones, is still low enough that there may not be any crashes in a given work zone because the exposure period is just too short to allow for statistically valid results. Finally, the problem is compounded by the fact that work zones differ in the way they treat maintenance of traffic. For example, some work zones use permanent barriers, while others use cones or drums; some narrow the lanes, while others maintain lane width and shoulders, etc.