7.3 - Congestion benefits

Introduction

Two types of congestion benefits for road users may result from transport service proposals:

• travel time benefits
• vehicle operating cost savings.

Nature of congestion benefits

Congestion benefits may be positive or negative.

In the presence of road congestion, the removal of some traffic will generally provide positive benefits to remaining road users. Some proposals, however, may achieve their improved transport service level by reducing the available road capacity for other road users. The level of congestion to remaining users may then be increased, creating a negative benefit.

Also congestion may be increased where a proposed transport service increases the number of passenger transport vehicles on roads shared with other traffic.

The effect of increased transport output on overall congestion will depend on:

• the change in the number of passenger transport vehicles per hour per period
• their size and performance characteristics
• the reduction in the number of trips
• the do minimum composition of road traffic flow.

Extent of analysis

Analysis must be undertaken for the transport service proposal and each option, compared against the do minimum.

Level of detail

The level of detail required for this analysis is determined by the size of the proposal. For large scale projects, it is considered important that the travel time benefits and vehicle operating cost savings are modelled to a reasonable degree of accuracy.

The information contained in this chapter will assist with the determination of the level of detail required.

Time periods

With respect to transport services, road user benefits shall generally be limited to peak periods. The proposer shall specify, and justify, the peak period times.

In some cases, for instance with most freight transport services, it may be appropriate to consider off-peak period road user benefits.

Variation of effects

It may be necessary to establish the benefits for different proposal years, if the do minimum road option is characterised by increasing congestion. Benefits or disbenefits may be estimated at five or ten year intervals. Intermediate years may then be interpolated.

Methods for travel time benefits

There are three basic approaches to assessing travel time benefits of transport service proposals on road users and other modes:

• Speed flow relationship.
• Modelling using procedures for evaluating roading project proposals (appendices A3 and A11 in volume 1). Appendix A11 will assist in determining the appropriate modelling to undertake and appendix A3 outlines the methods for calculating travel time saving benefits.
• Output using a validated transport model (worksheets A3.11 in volume 1 explain the validation process).

The choice of method depends on the magnitude of the corresponding road impact and the nature of the road(s) or road network affected by the introduction of the transport service.

If an accurate estimate of the benefits of reduced congestion is wanted, then the procedures in appendices A3 and A11 (volume 1) or output from a transport model should be used.

Definitions

The capacity of a road is the maximum flow rate at which vehicles can reasonably be expected to traverse a point under prevailing conditions.

A bottleneck is the point on a road section with the lowest capacity.

Choosing a method for travel time analysis

The following conditions identify when to use basic speed flow relationships for assessing congestion impacts, and when to use more detailed methods:

• If the case to be assessed consists of mainly arterial routes and flow rates are less than 85 percent of capacity (see note below) then use basic speed flow relationships.
• If the case to be assessed consists of a variety of road types or a complex road network and a variety of intersections, and bottlenecks with flows that are near to or over capacity then use the detailed procedures in appendix A3 and A11 (volume 1) or output from a validated transport model.

Note: If traffic flows are very near to or over capacity, during some period of the day, then it is advisable to use either the procedures in appendices A3 and A11 (volume 1) or the output from a validated transport model. This is because a small reduction in traffic flow could result in a significant reduction in queuing, which would be ignored if basic speed flow relationships were used.

Valuing travel time saving benefit

Once the change in travel time has been determined using one of the above methods, the value of the travel time savings is calculated using the appropriate values given in appendix A4 (volume 1). The increment for congestion (denoted as CRV) may be added to the base values for vehicle occupant time (table A4.1 of appendix A4 in volume 1) when the 'ruling' intersection or bottleneck of the corridor affected by the proposed transport service operates at least 80 percent capacity during the peak one hour period.

Note: Any increase in travel time is counted as a travel time disbenefit (negative benefit) and subtracted from the numerator of the BCR.

How to use speed flow relationships

Follow these steps to assess congestion impacts using basic speed flow relationships:

Note:  Steps 6 to 8 may also be used in calculating the value of travel time savings benefits when the change in travel time has been estimated using the procedures contained in appendices A3 and A11 (volume 1) or with output from a validated transport model.

In congested urban areas, removing road traffic will smooth flows and tend to reduce energy consumption and, to a lesser extent, the wear and tear on vehicles (tyres, clutch, brake blocks, etc). Outside urban areas, where average speeds exceed 70km/h, reducing speeds may reduce vehicle operating costs (VOC).

Vehicle operating cost benefits may be estimated as being equal to 5 percent of the value of travel time benefits for the same trips.

Note: All VOC benefit calculations use the predicted changes in average speed, as determined from the calculation of the travel time benefits