Section 3

Road User Cost Computations


3.1  COMPUTATIONAL ANALYSIS

Once the individual work zones have been identified, each is evaluated separately. This is the point at which individual road user cost components are quantified and converted to dollar cost values. The following sections provide an approach for quantifying and costing the individual road user cost components encountered. The potential work zone related road user cost components were discussed in the previous section and are listed below.

  1. Speed Change VOC
  2. Speed Change Delay
  3. Work Zone Delay
  4. Stopping VOC
  5. Stopping Delay
  1. Queue Delay
  2. Queue Idling VOC
  3. Circuity VOC
  4. Circuity Delay
  5. Crash Costs
The road user cost components shown as bold are computed fairly accurately and account for the majority of the total road user costs. Computations for each bolded component will be discussed in detail in the following sections. Worksheets (xls 53k) have been developed to aid in organizing and calculating the necessary data and are included in Appendix C (pdf 27k). It is recommended that these worksheets be used for the actual computations.

3.2   ANALYSIS OF THE WORK ZONE

The traffic demand and the capacity of individual work zones are important parts in calculating work zone related road user costs. Worksheet 3.1 (pdf 8k) has been developed to aid the analyst in comparing the traffic volume to the available capacity for each hour of the day. Worksheet 3.1 (pdf 8k) will provide the total affected traffic to be used in the computations and is discussed below.

Work Zone

Provides all relevant information pertaining to each work zone operation such as the number of lanes closed, the direction of travel, the day(s) of the week and the hours the work zone is in place.

Directional ADT & Year

The current or future directional ADT, based on the desired construction year, should be obtained from the traffic monitoring section. If future ADT is not readily available, the following formula can be used:

  (Future Yr. - Base Yr.)
  Future Year ADT = Base Year ADT x (1 + Growth Rate)

 

Percent Trucks & Cars

Provide the percent of each vehicle class that is present in the traffic stream.

Normal / Work Zone / Detour Capacity

The appropriate capacity is related to the allowable lane closure schedule and each is discussed in the Roadway Capacity section.

Lanes Under Normal Operation

The lanes under normal operation are the available number of traffic lanes per direction when no roadway restrictions are present.

Time Period - Column 3.1 (A)

The time period generally is shown as one-hour intervals over a 24-hour period. Intervals less than one hour can be used and require that the hourly traffic distribution and roadway capacity be revised accordingly.

Hourly Traffic - Column 3.1 (B)

The hourly traffic percent distribution can be determined from traffic data obtained from the traffic monitoring section. If such data can not be determined, the default hourly traffic percentages for various roadway classes (Statewide Averages (xls 20k) and by County (xls 200k)) can be used and are found in Appendix B (pdf 657k).

Vehicle Demand - Column 3.1 (C)

The hourly vehicle demand is calculated by multiplying the directional ADT by the hourly traffic percent distribution.

Lanes Open - Column 3.1 (D)

The number of lanes open on the facility varies directly with the allowable work zone lane closure hours provided in the contract.

Roadway Capacity - Column 3.1 (E)

Capacity is the maximum number of vehicles passing a point on the facility at established roadway conditions. In analyzing work zone related road user costs, there are three possible capacities that could be utilized and need to be determined. They include: (1) the capacity of the facility under normal operating conditions, (2) the capacity of the facility when the work zone is in place, and (3) the capacity of the facility to dissipate traffic from a queue condition. Each of these is discussed in turn.

(1) Normal Capacity

Normal Capacity is the maximum traffic volume a facility can handle under normal roadway conditions. Table 3.1 provides the ideal capacity a facility type can handle. Chapter 3 of the Highway Capacity Manual (HCM) points out that these capacities under ideal conditions must be adjusted for such real world factors as restricted lane widths, reduced lateral clearances, the presence of trucks and recreational vehicles, and the presence of a driver population unfamiliar with the area. The normal capacity of the facility is used during the non-work zone hours when all traffic lanes are open.

Table 3.1

Capacity by Facility Type

Facility Type Ideal Capacity
Freeway - 4 lanes 2,200 Passenger Cars per hour per lane
Freeway - 6 or more lanes 2,300 Passenger Cars per hour per lane
Multilane Highway 2,200 Passenger Cars per hour per lane
Two-Lane Highway 1,400 Passenger Cars per hour per lane (*)
Signalized Intersection 1,900 Passenger Cars per hour of green per lane

Source: 1994 HCM Table 2-14

(*) For 50/50 volume split by direction

(2) Work Zone / Detour Capacity

Capacity in the work zone can be estimated from research studies. Table 3.2 reflects average vehicle flow capacities at several real world work zones under several lane closure scenarios. These average capacities are 50% reliable. This means that the work zone capacity will be at least equal to the Table Value 50% of the time. On the other hand it also means the capacity of the work zone will be less than the Table Value 50% of the time.

Table 3.2

Measured Work Zone Capacities - Freeway Section

Number of
Directional Lanes
Number
of
Studies
Average Capacity Recommended
Value(*)
veh/lane/hour
Normal Open vehicles
per hour
vehicles per
lanes per hour
3 1 7 1,170 1,170 1,200
2 1 8 1,340 1,340 1,300
5 2 8 2,740 1,370 1,400
4 2 4 2,960 1,480 1,500
3 2 9 2,980 1,490 1,500
4 3 4 4,560 1,520 1,500

Source: 1994 HCM Fig. 6-11, Fig. 6-12, and Table 6.1

(*) Value may be increased 100 veh/lane/hour when the work zone is protected with Jersey barrier.

The recommended values in Table 3.2 are based on average capacities and a freeway type of facility. These values can be adjusted to reflect other facility types and changes in reliability (i.e., as the reliability increases the capacity decreases). The appropriate work zone and/or detour capacity would be used during the hours that lane restrictions are permitted.

(3) Dissipation Capacity

Capacity during queue dissipation may be less than the capacity during normal conditions, even though the lanes are unrestricted. According to the Highway Capacity Manual "various observations of freeway queue departure rates range from as low as 1500 pcphpl to as high as 2000 pcphpl". When compared to a capacity of 2000 pcphpl, this effect ranges from a significant reduction in capacity of 25% to virtually no reduction at all. This implies that a separate and distinct temporary "dissipation capacity" may exist after a work zone is removed. The appropriate dissipation capacity would only be used during the hours all traffic lanes are open with a physical queue present.

Queue Rate - Column 3.1 (F)

The queue rate is the difference between hourly capacity of the facility and the unrestricted hourly demand (demand minus capacity) during each hour of the day. The queuing rate is the hourly rate at which vehicles accumulate to, or, if negative, dissipate from any queue that may exist. A physical queue develops when the queue rate is greater than zero.

Queued Vehicles - Column 3.1 (G)

The number of queued vehicles are the vehicles "backed up" in the queue at the end of each hour. Once a queue develops, the number of queued vehicles equals the queue rate at the end of the first hour. The next hours' queue rate is then added to the previous queued vehicles total until all the queued vehicles have been dissipated.

Average Queued Vehicles - Column 3.1 (H)

The average number of queued vehicles in the queue for each hour is computed by averaging the number of queued vehicles at the beginning and end of each hour.

Vehicles That Travel Work Zone - Column 3.1 (I)

Under unrestricted flow conditions, the number of vehicles that travel the work zone is generally the traffic demand on the facility during the hours the work zone is in place. Under forced flow conditions, the number of vehicles that travel the work zone is limited to the capacity of the work zone.

Vehicles That Travel Detour - Column 3.1 (J)

The number of vehicles that travel the detour are those vehicles that are forced to use the alternate route during the hours the detour is in effect.

Vehicles That Travel Queue - Column 3.1 (K)

A physical queue develops when demand exceeds capacity (i.e. queue rate greater than zero). All vehicles that approach the work zone when a physical queue exists must stop and work their way through the queue before entering the work zone. Traffic that arrives as the queue starts to develop will have a rather short queue to work through, while traffic arriving when the queue is fully developed will have a much longer queue to travel. On the other hand, vehicles arriving as the queue is dissipating will have a continually shrinking queue to deal with. It is important to note that since the facility is operating under forced flow condition, the hourly volume of vehicles traveling the queue is limited to the capacity of the work zone. This is because the only way out of the queue is through the work zone.

Once the analysis of the work zone has been completed, the 24-hour totals should be entered for Hourly Traffic, Vehicle Demand, Vehicles That Travel Work Zone, Vehicles That Travel Detour, and Vehicles That Travel Queue. The affected traffic has now been determined and the analyst should now identify the road user cost components to be computed.

3.3   QUEUE DELAY

The queue delay per vehicle is only computed during forced flow conditions. Before computing the actual road user cost, the delay time through the queue (if applicable) must be known. Although the number of vehicles that travel the queue has been determined, the amount of delay can only be computed after knowing the queue length and queue speed. It is therefore necessary to determine the queue length and queue speed for each time period where a queue exists.

The delay time through the queue is determined by subtracting the time it takes to travel the queue length when it is present, from the time it takes to travel the same distance when it is not present. Worksheet 3.2 (pdf 8k) has been developed to aid the analyst in computing the overall queue delay per vehicle and is discussed below.

Queue Period - Column 3.2 (A)

A queue period is generally the hours a physical queue exists. It is not uncommon to have several queue periods within a 24-hour period. A physical queue develops when vehicle demand exceeds the roadway capacity and continues until all queued vehicles have been dissipated. The actual queue period(s) can be determined from the results of Worksheet 3.1 (pdf 8k).

Queue Volume - Column 3.2 (B)

The only way for traffic to exit the queue is through the work zone and therefore the volume through the queue section is limited to the capacity of the work zone.

Normal Capacity - Column 3.2 (C)

The normal capacity of the facility is the capacity of the roadway section operating in an unrestricted flow condition.

V/C Ratio - Column 3.2 (D)

The volume to capacity (V/C) ratio is calculated by dividing Column 3.2 (B) by Column 3.2 (C) for each queue period.

Average Queue Speed - Column 3.2 (E)

The average queue speed for each queue period is determined by using the V/C Ratio and the graph in Figure 3.1 shown below.

Unrestricted Speed - Column 3.2 (F)

The unrestricted speed of the facility is generally the posted speed limit of the section operating in an unrestricted flow condition.

Average Queued Vehicles per Queue Period - Column 3.2 (G)

The average number of queued vehicles for each queue period is computed by averaging the Average Queued Vehicles (Worksheet 3.1 (pdf 8k)) over the duration of each queue period.

Queue Lanes - Column 3.2 (H)

The number of queue lanes for each queue period is the available lanes upstream of the work zone that are used by vehicles when a physical queue develops.

Average Vehicle Length - Column 3.2 (I)

The average vehicle length for each queue period includes an assumed vehicle length (VL) and the space between vehicles. The mixed flow VL is 25 feet. The space between vehicles is computed as one VL for every 10 mph of queue speed. Therefore, a general rule-of-thumb is used: "The average vehicle length = One VL + One VL for every 10 mph of queue speed". For queue speeds up to 6 mph, use an average vehicle length of 40 feet.

Average Queue Length - Column 3.2 (J)

The average queue length for each queue period is computed by multiplying Column 3.2 (G) with Column 3.2 (I) and then dividing by Column 3.2 (H) and 5280 feet/mile.

Queue Travel Time At Unrestricted Speed - Column 3.2 (K)

This is the time necessary to travel the average queue length at the unrestricted speed and is computed by dividing Column 3.2 (J) by Column 3.2 (F).

Queue Travel Time At Queue Speed - Column 3.2(L)

This is the time necessary to travel the average queue length at the average queue speed and is computed by dividing Column 3.2 (J) by Column 3.2 (E).

Added Time To Travel Queue - Column 3.2 (M)

The added time to travel the queue for each queue period is computed by subtracting Column 3.2 (K) from Column 3.2 (L).

Affected Vehicles per Queue Period- Column 3.2 (N)

The affected vehicles per queue period is the number of vehicles that travel the queue during that period and is only required if there is more than one queue period in 24 hours.

Added Time per Queue Period - Column 3.2 (O)

The added time per queue period is computed by multiplying Column 3.2 (M) and Column 3.2 (N) and is only required if there is more than one queue period in 24 hours.

Added Time Weighted Average

The added time weighted average is computed by dividing the total of Column 3.2 (O) by the total of Column 3.2 (N).

3.4   QUEUE IDLING VOC

The queue idling VOC is only computed during forced flow conditions. At this point, an overall queue delay per vehicle has been determined. The queue idling VOC is computed by multiplying the number of vehicles that travel the queue, the overall queue delay per vehicle, and the current idling cost rate associated with "stop and go" driving in the queue. The current idling cost rate is computed in Section 3.7.

3.5   WORK ZONE & CIRCUITY (DETOUR) DELAYS

Before computing the actual road user cost, the delay time through both the work zone and detour (if applicable) must be known. Although the number of vehicles delayed through the work zone and/or the detour have been determined, the amount of delay can only be computed after knowing the work zone and/or detour lengths and the times through them. The circuity delay is only computed when a formal detour route has been established. The delay time through the work zone and through the detour are computed in the same manner. In each case, the delay is determined by subtracting the time it takes to travel either the work zone and/or detour when they are present, from the time it takes to travel the same distance when they are not present. Worksheet 3.3 (pdf 8k) has been developed to aid the analyst in computing the Work Zone and Circuity delays and is discussed below.

Work Zone Length - Column 3.3 (A)

The work zone length is generally the length of lane restrictions including transitions.

Work Zone Speed - Column 3.3 (B)

The work zone speed is generally a 10 mph-15 mph reduction in the unrestricted speed.

Unrestricted Speed - Column 3.3 (C)

The unrestricted speed of the facility is generally the posted speed limit of the section operating in an unrestricted flow condition.

Work Zone Travel Time At Unrestricted Speed - Column 3.3 (D)

This is the time necessary to travel the work zone length at the unrestricted speed and is computed by dividing Column 3.3 (A) by Column 3.3 (C).

Work Zone Travel Time At Work Zone Speed - Column 3.3 (E)

This is the time necessary to travel the work zone length at the work zone speed and is computed by dividing Column 3.3 (A) by Column 3.3 (B).

Added Time To Travel Work Zone - Column 3.3 (F)

The added time to travel the work zone is computed by subtracting Column 3.3 (D) from Column 3.3 (E).

Travel Length Without Detour - Column 3.3 (G)

This is the existing travel length prior to any lane restrictions or road closures.

Travel Length With Detour - Column 3.3 (H)

This is the length of the proposed detour or alternate route.

Added Travel Length - Column 3.3 (I)

The added travel length is computed by subtracting Column 3.3 (G) from Column 3.3 (H).

Travel Time Without Detour - Column 3.3 (J)

This is the time necessary to travel the existing facility prior to the proposed detour.

Travel Time With Detour - Column 3.3 (K)

This is the time necessary to travel the detour route.

Added Time To Travel Detour - Column 3.3 (L)

The added time to travel the detour is computed by subtracting Column 3.3 (J) from Column 3.3 (K).

Alternating Traffic (Flagging) Delay can be determined by utilizing Worksheet 3.3 (pdf 8k). The delay is determined by subtracting the time it takes to travel the flagging zone when it is present, from the time it takes to travel the same distance when it is not. Flagging work zones are generally up to a 1/3 mile in length and limited to a travel speed of 25 mph.

An additional component necessary to determine the road user cost associated with an Alternating Traffic (Flagging) work zone is the amount of time the approach vehicles are delayed while the opposing approach vehicles are traveling the flagging zone. The approach vehicle wait time should be based on the number of arriving vehicles during a given period and should not exceed five minutes per vehicle.

3.6   CIRCUITY (DETOUR) VOC

The circuity VOC is only computed when a formal detour route has been established. At this point, an overall added travel length per vehicle has been determined. The circuity VOC is computed by multiplying the number of vehicles that travel the detour, the overall added travel length per vehicle, and the current VOC cost rate associated with driving the added distance. The current VOC cost rate is computed in Section 3.7.

3.7   ESCALATION FACTORS & COST RATES

The National Cooperative Highway Research Program (NCHRP) Report 133, Procedures for Estimating Highway User Costs, Air Pollution, and Noise Effects, provides 1970 travel time values of $3.00/hour for cars and $5.00/hour for trucks. The 1970 Idling Costs are provided in Table 5 of the same report and the average 1970 VOC values are derived from Figures A-13, A-14, and A-15 of the same report. The 1970 values are generally accepted to provide reliable costs although changes may have occurred regarding trip purpose and driving habits.

The CPI-U is an accepted escalation tool and the values are available in Almanacs or at the Bureau of Labor Statistics web site. Also, in the event that research studies update or supercede the NCHRP Report 133 values, the use of the CPI-U for escalation purposes remains applicable. Worksheet 3.4 (pdf 8k) has been developed to aid the analyst in computing the appropriate escalation factors and current cost rates and is discussed below.

Cost Factors - Column 3.4 (A)

The Idling and VOC cost factors are based on the "Transportation Component" of the CPI-U. The Time Value cost factor is based on the "All Items Component" of the CPI-U.

1970 (CPI-U) - Column 3.4 (B)

The "Transportation Component" of the CPI-U was 37.5 in 1970. The "All Items Component" of the CPI-U was 38.8 in 1970.

Current (CPI-U) - Column 3.4 (C)

The "Transportation Component" and "All Items Component" of the CPI-U should be determined for the current or desired year.

Escalation Factor - Column 3.4 (D)

The escalation factor for each cost factor is computed by dividing the Current CPI-U in Column 3.4 (C) by the 1970 CPI-U in Column 3.4 (B).

Vehicle Class - Column 3.4 (E)

Section 1.7 of this manual stated that 13 different vehicle classifications exist. "Car" and "Truck" classifications are only considered in the road user cost computations.

1970 Time Value Cost Rate - Column 3.4 (F)

NCHRP Report 133 reflects 1970 travel time value cost rates as $3.00/hour for cars and $5.00/hour for all trucks.

1970 Idling Cost Rate - Column 3.4 (G)

NCHRP Report 133 reflects 1970 Idling cost rates as $0.1819/veh-hr for cars, $0.2017/veh-hr for single unit trucks, and $0.2166/veh-hr for combination trucks. The cost rate for single unit trucks and combination trucks has been averaged to reflect $0.2092/veh-hr for all trucks.

1970 VOC Cost Rate - Column 3.4 (H)

NCHRP Report 133 reflects average 1970 VOC cost rates as $0.06/mile for cars, $0.09/mile for single unit trucks, and $0.14/mile for combination trucks. The cost rate for single unit trucks and combination trucks has been averaged to reflect $0.12/mile for all trucks.

Current Time Value Cost Rate - Column 3.4 (I)

The current time value cost rate for each vehicle class is computed by multiplying the 1970 time value cost rate in Column 3.4 (F) by the Time Value escalation factor in Column 3.4 (D).

Current Idling Cost Rate - Column 3.4 (J)

The current idling cost rate for each vehicle class is computed by multiplying the 1970 idling cost rate in Column 3.4 (G) by the Idling escalation factor in Column 3.4 (D).

Current VOC Cost Rate - Column 3.4 (K)

The current VOC cost rate for each vehicle class is computed by multiplying the 1970 VOC cost rate in Column 3.4 (H) by the VOC escalation factor in Column 3.4 (D).

Table 5 from NCHRP Report 133 is also designed to determine speed change costs. Speed change costs are the added costs (VOC and Delay) of slowing/stopping from one speed to another and returning to the original speed. Speed change costs are calculated by subtracting the cost and time factors of slowing/stopping at one speed from the cost and time factors of slowing/stopping at another speed. The road user cost components associated with slowing/stopping are discussed in Section 2 and will not be computed in this manual.

3.8   ROAD USER COSTS

At this point, all the necessary data to compute road user costs has been compiled. It is now necessary to distribute the traffic impacted by the various road user cost components to the appropriate vehicle classes. Worksheet 3.5 (pdf 8k) has been developed to aid the analyst in computing the road user costs and is discussed below.

Road User Cost Component - Column 3.5 (A)

There are ten potential work zone related road user cost components that can occur. The five components shown are generally computed fairly accurately and account for the majority of the road user costs.

Vehicle Class - Column 3.5 (B)

"Car" and "Truck" vehicle classifications are only considered for each road user cost component.

Percent Class - Column 3.5 (C)

The percent of each vehicle class in the traffic stream should be part of the data obtained from the traffic monitoring section. The percent class is entered at the bottom left corner of the worksheet and for each appropriate road user cost component.

Total Vehicles - Column 3.5 (D)

The number of total vehicles that travel the queue, work zone or detour over a 24 hour period are obtained from Worksheet 3.1 (pdf 8k). The total vehicles are entered at the bottom left corner of the worksheet and for each appropriate road user cost component.

Added Travel Length - Column 3.5 (E)

The appropriate value for added travel length is obtained from Worksheet 3.3 (pdf 8k).

Added Time - Column 3.5 (F)

The appropriate values for added time are obtained from Worksheet 3.2 (pdf 8k) & Worksheet 3.3 (pdf 8k).

Cost Rate - Column 3.5 (G)

The appropriate cost rates for time, idling, and VOC are obtained from Worksheet 3.4 (pdf 8k).

Road User Cost - Column 3.5 (H)

The road user cost associated with each component is obtained by multiplying the values across each row for each vehicle class and rounding to the nearest dollar.

Daily Road User Cost

The daily road user cost is computed by totaling the road user costs for each component./P>

Calculated Road User Cost (CRUC)

The calculated road user cost is computed by multiplying the daily road user cost by a 50% reduction factor. The reduction factor is used to accommodate for variations in traffic data, roadway capacities, and cost rates.

Number of Work Zone Days

The number of work zone days is the anticipated number of days that the work zone or detour will be in effect.

Total Road User Cost

The total road user cost is computed by multiplying the calculated road user cost by the number of work zone days.


Last Document Correction:
March 15, 2006