How Much Road Capacity is It Appropriate to Provide? Lessons from the South and West Yorkshire Multi Modal Study

How Much Road Capacity is It Appropriate to Provide? Lessons from the South and West Yorkshire Multi Modal Study


A Skinner, MVA; D Coombe, The Denvil Coombe Practice; Representative from the Government Office for Yorkshire and the Humber, UK



Existing inter-urban roads are congested and traffic levels are forecast to grow. Congestion is seen as damaging to the economy of a depressed area. High levels of investment in alternatives to car travel and road haulage will have only a modest impact on existing or future levels of congestion.

In these circumstances how much new road capacity is it appropriate to provide? On what rational basis can judgement be made? Should we attempt to maintain a defined level of service? Should capacity be increased to the point where economic benefits are equal to direct and external costs? What level of traffic should we be designing for, and how much 'slack' should be built in for future growth? How can the concept of sustainability be operationalised?

These were the dilemmas faced by the consultants and the Steering Group for the South and West Yorkshire Multi-Modal Study (SWYMMS). This paper describes how these complex issues were addressed for the strategic highway network within and between two of the UK's largest urban areas.

The South and West Yorkshire Multi-Modal Study (SWYMMS) is one of several Roads-Based and Multi-Modal Studies recently undertaken by the UK Department for Transport (DfT). Multi-Modal Studies examine problems of congestion on the strategic road network and seek solutions from all modes of transport. In Spring 2000, the Government Office for Yorkshire and the Humber (GOYH) commissioned a consortium led by MVA Limited to undertake SWYMMS. The Study Area covers much of South and West Yorkshire, including all the main urban areas. The overall aims of SWYMMS were to make recommendations for:

* an integrated and sustainable strategy for the strategic road, rail and water networks in the Study Area; and

* a plan of specific interventions to address the most urgent key strategic problems in the Study Area through to 2021.

The study-specific objectives which the strategy and plan should achieve were:

* to reduce congestion on the motorways and A1;

* to re-establish the primary role of the trunk road network for strategic traffic;

* to facilitate sustainable economic regeneration of depressed areas, especially the Objective 1 status area of South Yorkshire and the Objective 2 status areas of West Yorkshire; and
* to sustain economic growth in other parts of the Study Area.

Bearing in mind the origins of the Study in the 1998 Trunk Roads Review (A New Deal for Trunk Roads in England, DETR), and the emphasis in the study-specific objectives on the motorways and A1, the study concentrated it's investigations of alternative modes on the potential for reducing motorway and A1 congestion.

The Study established that the causes of current and future congestion on the motorways and A1 in the Study Area are:

* the congested sections of motorways and A1 in the peak periods are used by large numbers of commuters;

* the average occupancy of cars used for commuting is only 1.15;

* some of these car commuters are travelling very long distances;

* many of their destinations as well as their origins are widely dispersed, and only a small proportion end in town or city centres;

* for many of these car commuters to use the current public transport system, they would incur substantial wait times for infrequent services and would have to interchange between services to a degree likely to be regarded as unacceptable; and

* much of the current rail system is overloaded in the peaks, and could not carry many of the car commuting movements without capacity enhancements.

Following an analysis of transport and land-use/economic strategies for the region, to guide our work, we agreed four key principles with the Steering Group. The thought process behind their development was that standard appraisal procedures deal only with circumstances in a particular year, and not with implications of actions over time, and this is inherently unsuitable for dealing with the issue of sustainability. At this point there was considerable cross fertilisation of ideas between SWYMMS and a similar study in London examining the M25.

The First Key Principle is that broad categories of measure should be investigated in the following order:

* demand management;
* alternatives to the car; and
* new road capacity, as a last resort.

This approach is in line with the Transport White Paper (A New Deal for Transport: Better for Everyone) produced by the Department for the Environment, Transport and the Regions in 1999).

The Second Key Principle is that any proposed increase in road capacity, however achieved, should be accompanied by proposals for means of controlling the use made of the total new road capacity, i.e. some form of demand management. The rationale for this principle is that, without controls, traffic will grow to use up the road space provided and the levels of service on the new or improved road will rapidly deteriorate, to the point where further capacity is required if congestion is to be avoided. The intention is that the benefits of the increased capacity should be 'locked in' as far as possible so that they cannot be eroded by induced traffic (sometimes called 'generated' traffic).

The reasons why traffic levels increase over time and in response to new road capacity have been explained by the Standing Advisory Committee on Trunk Road Assessment in their 1994 Report on Trunk Roads and the Generation of Traffic and their 1999 Report on Transport and the Economy.

The Third Key Principle aims to avoid the over-provision of road capacity and also provides for increased sustainability. The basis is that the traffic levels which should be used for the development of proposals to increase road capacity should be those that would arise from an economically optimum area-wide road user charging regime in the year in which such a regime first showed a positive economic rate of return and is practical. The traffic flows at that year under that optimum charging regime should form the basis for the development and appraisal of capacity increases to the motorways, whether or not road user charging is implemented. The rationale for this approach is that we considered that it would not be wise to undermine the case for area-wide road user charging by over-providing road capacity.

The Fourth Key Principle is that schemes to increase road capacity should generally be considered only where, with the relevant forecast traffic flows, average traffic speeds in peak hours on the motorways and A1 would be 55 mile/h or below.

Following the First Key Principle we established that the only practical method for reducing demand for travel on the motorways and the A1 would be a system of area wide road user charging. This would be both technically feasible and economically viable in 2010. Government policy is that there are no plans to introduce such a scheme for cars, and such proposals would not be implemented within the period up to 2010. We also established that even implausibly large levels of investment in alternatives to cars and road haulage would not impact significantly on motorway congestion.

Some new road capacity was therefore deemed to be required, the question remaining was 'How Much?'.

Investigations relating to the Second Key Principle established that measures such as motorway charging or ramp metering can, to a significant degree, be used to 'lock in' the benefits of any capacity increase, by discouraging induced traffic.

The Third Key Principle provided the basis for setting the level of recommended road capacity increase. Extensive tests established economically optimum road user charges and traffic flow levels for 2010. Using the criteria from the fourth Key Principle schemes were identified and appraised on the basis of these flows, using standard UK criteria (New Approach To Appraisal).

The most important recommendations from the Study were as follows:

* the motorways and A1 should be widened, but only with integral measures to minimise induced traffic such as ramp metering, the aim was to achieve a higher level of service, not more traffic travelling at current speeds; and
* area-wide road user charging should be introduced as soon as possible after 2010.

Tests of the proposed capacity increases were carried out using forecasts for 2020. With economically optimum road user charging in place the plan was robust, with only a small number of links experiencing travel conditions worse than those defined in the Fourth Key Principle. There would be scope for dealing with these through minor variations to the charging regime, which had not been designed in detail. Without charging, congested conditions are forecast to gradually return over the period up to 2020. This is because the integral demand management measures have to be relatively weak in nature, to avoid traffic diversion to less suitable roads. In such circumstances the Study Area will be faced with a choice between congestion and a further round of inherently unsustainable capacity increases.

At the time of submission of this abstract the UK Government is considering the report of the South and West Yorkshire Multi-Modal Study.


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