Future scenarios and strategic choices - Transport Policy and Planning in Great Britain - page 419

it should be emphasised, do not incorporate any deterrent effects of potentially greater
congestion. In effect they represent levels of demand assuming the conditions for
travel remain the same as today. (In practice they could therefore be more or less
depending on the policy response.)
Road users already adapt their behaviour in response to traffic conditions. This
has resulted in the progressive spreading of peak periods such that the scope for
further adaptation is becoming more limited. The authors acknowledge the variety of
measures which can be introduced to reduce car use, including conventional demand
management, improvement of public transport, and the range of 'smarter choices':
All have potential for reducing road use cost-effectively in suitable locations
and circumstances but the actual reduction is small in relation to total road use.
Suitable locations for many of these measures tend to be in urban areas and on
local roads so that they do not greatly affect traffic on the main road network.
(ibid. Chapter 4)
Likewise there is a range of measures available for managing traffic on main roads,
especially motorways but again the authors claim that their impact on the overall
situation is small:
Traffic engaged in longer distance trips relies to a large extent on the motorways
and trunk roads which have become progressively more crowded. Here additional
capacity generally offers the best means of improvement. As current opportunities
for widening and junction improvements are implemented the potential for
increasing the capacity of existing strategic roads will diminish and consequently
the case for building more new roads will strengthen.
The Study tested options for inter-urban road building, with and without pricing,
against a base scenario which assumed completion of the Highways Agency's Targeted
Programme of Improvements projected to 2015 (an extra 1,594 lane km in England),
no increase in road capacity after that and no pricing.
Scenarios for the period 2010-41 consisted of additional capacity at the rates of
200, 400, 600 and 800 lane kilometres per annum (Lkmpa) with junctions improved
to match the capacity of links. The capacity is allocated approximately 60:40 between
motorways and trunks roads and 30:70 between conurbations and other areas, but
varied between regions. In the 800 Lkmpa scenario the extra 200 Lkmpa is confined to
the fastest growing regions only.
The pricing scenario applied a national charging scheme with rates reflecting the
'marginal cost to society', i.e.
• a rate per vehicle km for the cost of providing and maintaining the roads and
for environmental and safety impacts which vary by vehicle type, road type and
degree of urbanisation, and
• a rate for congestion which varies with traffic conditions, time of day and day of
the week to reflect the additional delay imposed on other roads users and carbon
emissions (zero in uncongested situations).
The results of these scenarios in terms of their effect on flows and speeds in 2041
relative to the base case are shown in Figure 24.2. Without a national charging scheme,