68 Highway Engineering
table’s standard seven-point scale (neutral, slight, moderate or large benefi-
cial/adverse) is utilised. In situations where the scheme is unacceptable in
terms of nature conservation, the assessment of ‘very large negative’ can be
applied.Heritage
This criterion assesses the impact of the proposal on the historic environment.
It too is assessed on the AST’s standard seven-point scale.Water
In order to gauge the effect of the proposal on the water environment, a risk-
based approach is adopted to assess its potentially negative impact on both
water quality and land drainage. Both these are evaluated on a three-point scale
of high/medium/low in an effort to gauge the overall sensitivity of the water
environment. The potential of the proposal to cause harm is then determined
using two indicators:
Tr affic flows – relating to water quality
The surface area of the proposal (total land take) – relating to land
drainage/flood defence.Again, for this stage, the same three-point assessment scale is used
(high/medium/low). In relation to water quality, traffic flows in excess of
30 000 annual average daily traffic (AADT) are assessed as having a ‘high’ poten-
tial to cause harm, with flows between 15 000 and 30 000 AADT assessed as
‘medium’ and those less than 15 000 AADT assessed as ‘low’. For land
drainage/flood defence, areas in excess of 40 ha are assessed as having a ‘high’
potential to cause harm, with areas between 10 and 40 ha assessed as ‘medium’
and areas less than 10 ha assessed as ‘low’. Based on the information from both
stages, an assessment using only the neutral/negative points on the AST’s
assessment scale is used to indicate the proposal’s overall performance on this
criterion.Safety
This criterion measures the extent to which the proposal improves the safety for
travellers, indicating its effectiveness in terms of the monetary value, in present
value terms, of the reduction in accidents brought about directly by the con-
struction of the new/improved road. This requires accidents to be broken down
into those causing death, those causing serious injury and those resulting in only
slight injury. The results for this criterion can be obtained directly from COBA.
The discount rate used is 6%, with all values given in 1994 prices, and it includes
accidents likely to occur during both the construction and maintenance phases
of the proposed road.