| Contextual Infrastructure Planning and Design |
| Flexibility in the Geometric Design of UK Highways |
| By Ian Wilson, Cardiff, Wales 0144 2920 396045, Wilsoni@pbworld.com
|
| This article presents several
examples of flexible highway design in the UK and a discussion how
UK standards control this flexibility. |
|
Flexible design of highways has been used in the UK for many years,
examples from the Cardiff office being the Glyn Bends Improvement
(described in an article that follows by Jeff Rowlands) and the improvement
of Ceredigion Links Roads to the north of Carmarthen. The UK design
approach was developed to minimise costs whilst providing a set of
design standards that did not compromise safety.
A further example is the A479 Talgarth Relief Road project in mid
Wales, which is at the preliminary design stage. The initial proposal
was for a 4-km (2.4-mile) bypass to full standards, including the
provision of overtaking (passing) opportunities. The options currently
being examined, however, are to a lower geometric standard and are
more sympathetic to the local environment.
PB is also examining the widening of the dual two-lane section of
the M4 passing to the north of Cardiff. This is one of many lengths
of the motorway network in the UK that now require widening, but widening
is often more difficult than the initial construction because of the
constraints on space that developed in the interim. To deal with these
constraints, UK practice is to reassess the need for continuous hard
shoulders and to consider using narrower lanes where pinch points
exist.
"TD 9 Highway Link Design"
Highways design standards in the UK are brought together in the Design Manual for Roads and Bridges, a 14-volume collection of loose-leaf
documents consisting of "Departmental Standards," "Advice
Notes" and the older "Technical Memoranda." These documents
cover all aspects of design, including related subjects such as scheme
assessment and economics reporting and environmental assessment. In
general, they have been prepared and agreed to jointly by the Overseeing
Departments in England, Scotland, Wales and Northern Ireland.
In 1981 a new Departmental Standard, "TD 9 Highway Link Design,"
was first introduced, superseding the Layout of Roads in Rural Areas,
which was published in the mid 1960s. To designers, the latter was
always associated with the phrase "flowing alignments,"
whereas the new document encouraged roads with straighter lengths
and curves tighter than before to increase overtaking (passing) opportunities.
The other fundamental change affected the entire methodology of highway
design, giving designers considerable flexibility in the geometric
standards they could choose to adopt. The flexibility is controlled,
however, and does not result in designers having total freedom in
their work.
The concept of flexibility is based around the idea that there are
certain minimum standards that designers should aim for but that,
in certain situations, the cost of achieving these minimum standards
can be unreasonably high in either economic or environmental terms
when compared with the benefits that would accrue as, for instance,
in the case of a maximum gradients and summit curves. Further, safety
would have been an overriding concern prior to the introduction of
"Highway Link Design," but research has shown that safety
is not immediately compromised by a small or even moderate reduction
in standards. This is explained in the "Highway Link Design"
as follows.
"Studies carried out on rural all-purpose roads for
the development of this standard aimed to correlate personal injury
accident rates with horizontal curvature, gradient and sight distance.
Results were consistent with those of other studies, showing that
values of these layout parameters that are below desirable minimum
values were associated with slightly increased accident rates,
and that the increase did not become significant until the difference
from the desirable minimum value was significant."
Relaxations and Departures
The "Highway Link Design" sets out principal highway geometric
parameters-stopping sight distance, horizontal curvature and vertical
curvature-in tabular form for different design speeds. The desirable
minimum value is quoted, as are various other lesser standards such
as one or two steps below desirable minimum. These figures are related
in a hierarchy whereby the desirable minimum for one design speed
is the same as that for one step below desirable minimum for the next
greater design speed. Designers are permitted to deviate from the
desirable minimum standard by a varying number of steps in different
circumstances. A deviation of first tier significance is termed a
"relaxation" while a further deviation to a second tier
of significance is termed a "departure from standards."
Relaxations are generally permitted after consideration is given several
criteria, including to what extent the site, for example:
- Is isolated from other relaxations
- Is isolated from junctions
- Is one where drivers have Desirable Minimum
Stopping Sight Distance.
Departures are not precluded from a designer's range of options. It
is accepted that "in situations of exceptional difficulty that
cannot be overcome by relaxations, it may be possible to overcome
these situation by adoption of departures..." In theory, there
is a different procedure in that the overseeing department must give
approval to departures. In practice, however, designers need to list
possible relaxations in order to document that such relaxations have
been considered but are not appropriate to the situation. This listing
must be included in the Scheme Assessment Report, which the overseeing
department would ultimately approve. From the designer's view the
difference is, therefore, largely academic, although inevitably a
great deal of thought would be given to an application for a departure.
Whether a scheme includes departures or relaxations, the procedures
in the UK ensure that designers do not simply produce designs that
fail to meet standards, but rather that they show how they used professional
judgment in a thoughtful and intelligent manner to achieve an appropriate
solution to an engineering problem. |
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Ian Wilson is a Senior Engineer with 22
years experience in highway design. He has served as lead engineer
on a variety of large and small schemes ranging from feasibility studies
through preliminary design to detailed design, contract documents
and site supervision. |
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