Impact prediction, evaluation and mitigation - Introduction to Environmental Impact Assessment - page 145

significant extends over a range of values which build on perceptions as much as facts.
Socioeconomic impacts can raise in particular the distributional dimension to evaluation,
“who wins and who loses” (Glasson 2001, Vanclay 1999). Beyond the use of standards
and legal requirements, all assessments of significance either implicitly or explicitly
apply weights to the various impacts (i.e. some are assessed as more important than
others). This involves interpretation and the application of judgement. Such judgement
can be rationalized in various ways and a range of methods are available, but all involve
values and all are subjective. Parkin (1992) sees judgements as being on a continuum
between an analytical mode and an intuitive mode. In practice, many are at the intuitive
end of the continuum, but such judgements, made without the benefit of analysis, are
likely to be flawed, inconsistent and biased. The “social effects of resource allocation
decisions are too extensive to allow the decision to 'emerge' from some opaque
procedure free of overt political scrutiny” (Parkin 1992). Analytical methods seek to
introduce a rational approach to evaluation.
Table 5.4 Examples of standards in relation to
impacts of projects on traffic noise in Britain

• BS 7445 is the Standard for description and measurement of environmental noise. It is in three
parts: Part 1: Guide to quantities and procedures, Part 2: Guide to acquisition of data, and Part 3:
Guide to application of noise limits.
• Noise is measured in decibels (dB) at a given frequency. This is an objective measure of sound
pressure. Measurements are made using a calibrated sound meter.

• Human hearing is approximately in the range 0-140 dBA.
dB Example of noise
<40 quiet bedroom

60 busy office

72 car at 60 km/h at a distance of 7 m

85 Heavy goods vehicle (HGV) at 40 km/h at a distance of 7 m

90 hazardous to hearing from continuous exposure

105 jet flying overhead at 250 m

120 threshold of pain.

• Traffic noise is perceived as a nuisance even at low dB levels. Noise comes from tyres on the
road, engines, exhausts, brakes and HGV bodies. Poor maintenance of roads and vehicles and
poor driving also increase road noise. Higher volumes of traffic and higher proportions of HGVs
increase the noise levels. In general, annoyance is proportional to traffic flow for noise levels
above 55 dB(A). People are sensitive to a change in noise levels of 1 dB (about 25 per cent
change in flow).

• Assessment of traffic noise is assessed in terms of impacts within 300 m of the road. The EIA
will estimate the number of properties and relevant locations (e.g. footpatbs and sports fields) in
bands of distance from the route: 0-50 m, 50-100 m, 100-200 m, 200-300 m, and then classify
each group according to the baseline ambient noise levels (in bands of <50, 50-60, 60-70, >70
dB(A)) and the increase in noise (1-3, 3-5, 5-10, 10-15 and >15 dB(A)).