A4.4 Traffic congestion values

Introduction

Road users value relief from congested traffic conditions over and above their value of travel time saving. The maximum increments for congestion values apply to vehicle occupants. or road category and time periods as indicated in tables A4.1, A4.2 and A4.3. The actual additional value for congestion used in the evaluation is adjusted according to the requirements set out below.

Treatment of passing lane projects

An exception to the procedures below is made in the case of passing lane projects evaluated using the procedures in appendix A7 of this manual. The procedures in appendix A7 include a separate value for the reduction in driver frustration and the effect of reducing travel time variability. When evaluating passing lanes using the procedures in appendix A7, no additional allowance shall be made for congestion or improvements in trip reliability. Similarly, if passing lanes are evaluated using the values for congestion and/or reliability outlined in this appendix, then no allowance can be included for driver frustration.

Congested traffic conditions - rural 2 lane highways

To allow for congestion, the following addition should be made on sections of rural 2 lane highways. Section lengths for this analysis should normally be greater than 2 km.

Peak traffic intensity and volume to capacity ratio (VC ratio) are first calculated in the normal manner (see appendix A3.17). Using the VC ratio, terrain type and percentage no-passing for the road section, the percentage of time delayed (PTD) following slower vehicles is selected from figure A4.1 or table A4.4. Alternatively, the formulae shown in figure A4.1 can be used to calculate PTD, within a limiting range of PTD greater than or equal to 30 percent. For lower values of PTD the curves are linear.

Incremental value for congestion = CRV ×PTD/90 ($/h)

where CRV is the value for congestion (in $/h) and is given in table A4.1 for drivers or passengers, and in table A4.3 for standard traffic compositions.

Percentage of time delayed has a maximum limit of <90%, for situations where PTD is ≥90%, the maximum increment for congestion (CRV) should be added to the base value of travel time.

Congested traffic conditions - urban roads, multi-lane rural highways and motorways

To allow for congestion, the following addition should be made to road section travel time values where the time period VC ratio exceeds 70%.

Incremental value for congestion =

Bottleneck delay

For all bottleneck delay, the maximum increment for congestion from table A4.1 or table A4.3 should be added to the base value of travel time.

Worked examples

Four worked examples are given below of the calculations for the value of congestion. In each case, the example describes the calculation for a single time period and for the base year. For a full project evaluation, the calculations would be made for each flow period and for future year traffic forecasts as necessary.

Example 1 - Rural highway: realignment

A project involves the realignment of a busy 2 km section of rural highway, which improves sight distances, providing more overtaking opportunities for following traffic. The road is classified as rolling terrain.

From calculations in appendix A2 and/or A3, the road section carries 12,500 veh/day, with a peak interval intensity of 1,000 veh/h, 60/40 directional split and 12 percent heavy truck component. In the do minimum, the alignment offers no passing opportunities (0 percent overtaking sight distance), and after realignment there is no restriction on overtaking sight distance (100 percent overtaking sight distance). The hourly capacity of the road in the do minimum is calculated as:

2,800 × f_t × f_d = 2,800 × 0.675 × 0.94 = 1,775 veh/h

where: 2,800 is the ideal capacity of the road section; f_t and f_d are adjustment factors for directional distribution and the proportion of trucks (see appendix A3.11). The peak interval traffic intensity (1,000 veh/h) divided by capacity gives a VC ratio of 56 percent.

From figure A4.1(b), the PTD in the do minimum is 79 percent, and 71.5 percent after realignment. The maximum increment for congestion (CRV) for rural strategic roads is $4.23 per veh/h (from table A4.3).

The incremental values for congestion for the do minimum and project option are calculated as follows:

Do minimum: 4.23 × 79/90 = $3.71 per veh/h

Project option: 4.23 × 71.5/90 = $3.36 per veh/h

The time period total average travel time for the road section is calculated using the procedures in appendix A3.22 (based on component values calculated in other sections of appendix A3). For this example, the average travel times per vehicle have been calculated as 1.70 and 1.30 min/veh for the do minimum and realignment option, respectively.

The congestion cost savings are calculated by multiplying the peak interval traffic intensity by the incremental value for congestion and the time period average travel time divided by 60. For example:

Do minimum = 1,000 × 3.71 × 1.70/60 = $105.1/h

Project option = 1,000 × 3.36 × 1.30/60 = $72.8/h

Congestion cost saving = $105.1 − $72.8 = $32.3/h over the peak period.

Example 2 - Rural highway: 4 laning

A section of 2-lane rural strategic road is approaching capacity. One option is 4 laning. The road carries 20,000 veh/day in rolling terrain with 20 percent overtaking sight distance, peak interval traffic intensity of 2,050 veh/h, 70/30 directional split and 7 percent heavy truck component. The ideal capacity for a 2-lane rural road is 2,800 vehicles/hour (total in both direction of travel).

For the do minimum, the congestion cost is calculated in the same way as in example 1. The capacity is 2,800 × f_d × f_t = 2,800 × 0.89 × 0.92 = 2,290. This compares with a traffic volume of 2,050, which gives a VC ratio of 0.90. The percentage of time delayed is 90 percent from table A4.4. The incremental value of congestion is therefore equal to is the maximum incremental value of $4.23 per veh/h from table A4.3.

For the 4 laning option, assuming there are no restrictions requiring a reduction in the lane capacity, a capacity of 2,200 veh/h/lane is applicable (See appendix A3.10). The VC ratio is 2,050/(4 × 2,200) = 0.23, which is below 70 percent, so congestion costs are not applicable.

The saving in congestion costs over the peak period is $4.23 per veh/h multiplied by the section traffic volume and time period average travel time for the do minimum.

Example 3 - Urban arterial road: additional traffic lanes

A project provides a 4 lane clearway in the peak direction for an urban arterial road and improves the capacity of a signalised intersection half-way along the project length.

The morning peak interval traffic intensity is 1,000 veh/h in the peak flow direction (from appendix A3.16). Capacity has been established to be 1,250 veh/h for the do minimum and 2,000 veh/h with the clearway project (based on the multilane road capacity procedure in appendix A3). The road section VC ratio reduces from 80 percent to 50 percent as a result of the project.

Intersection stopped delay will be reduced from 15 s/veh in the do minimum to 6 s/veh after widening for the 2,000 veh/h through the intersection.

The incremental value of congestion for the road section in the do minimum for the peak direction of flow is given by:

where: $3.88 per veh/h is the CRV value from table A4.3.

With the clearway, the VC ratio in the peak direction is below 70 percent, so no incremental value for congestion is applicable. The congestion cost saving for the road section travel time is therefore $1.29 per veh/h multiplied by the traffic volume and average vehicle travel time for the section.

For the bottleneck delay, the incremental value for congestion is given by:

Do minimum = $3.88 ×15/3600 = $0.0162/veh through the intersection

Intersection improvement = $3.88 × 6/3600 = $0.0065/veh through the intersection.

Congestion cost saving per vehicle = $0.0162 − $0.0065= $0.0097/veh through the intersection.

The congestion cost saving attributable to reduction in bottleneck delay is $0.0097/veh multiplied by 2000 veh/h using the intersection = $19.40/h over the peak period

Example 4 - Urban intersection improvement

A project proposal will reduce delay and improve safety at a priority-controlled
T-intersection through the installation of a roundabout. Traffic volumes on the three approaches to the intersection are evenly balanced, there is a high proportion of turning traffic and the configuration of the site is such that a roundabout can be constructed without additional land take.

Bottleneck delay to side road traffic during the peak interval of the morning peak period has been observed to average 35 s/veh for the 500 veh/h on the side road approach, and 5 s/veh for the 300 veh/h turning off the main road. With the roundabout, traffic volume and bottleneck delay for the three approaches has been modeled at: 500 veh/h and 7 s/veh; 700 veh/h and 5.5 s/veh; and 600 veh/h and 6 s/veh.

Total bottleneck delay is calculated as:

Do minimum = (500 ×35 + 300 ×5) / 3600 = 5.28 veh/h

Roundabout option = (500 ×7 + 700 ×5.5 + 600 ×6) / 3600 = 3.04 veh/h

Reduction in bottleneck delay = 5.28 - 3.04 = 2.24 veh/h

Congestion cost saving = 2.24 ×CRV = 2.24 ×$3.88 = $8.68/h over time period.

Table A4.4(a) VC ratios for level terrain, overtaking sight distance and percentage of time delayed (PTD) following slow vehicles

Table A4.4(b) VC ratios for rolling terrain, overtaking sight distance and percentage of time delayed (PTD) following slow vehicles

Table A4.4(c) VC ratios for mountainous terrain, overtaking sight distance and PTD following slow vehicles

Figure A4.1(a) - Percentage of time delayed (PTD) two lane rural roads, level terrain

Figure A4.1(b) - PTD for two lane rural roads, rolling terrain

Figure A4.1(c) - PTD for two lane rural roads, mountainous terrain