The Route Selection Report
reviewed the Yellow and Blue options with alternative cross sections (wide
single carriageway and dual carriageway) for the section east of Macroom so
that the cost and economic benefits could be compared. The chosen cross-sections for the preferred
route corridor are as follows;
|
Node 1 to 5a |
West of Macroom |
Standard Single
Carriageway |
|
Node 5a to 7a |
Macroom Bypass |
Wide Single Carriageway |
|
Node 7a to 16 |
East of Macroom |
Wide Single Carriageway |
On all sections of the route, the minimum cross-section
has been used but the traffic flows, for most sections, exceed the capacity
defined for level of service D for rural routes. The traffic in the design year, 2024, is detailed in Figure 22.1. The
cross sections proposed are preliminary and will be reviewed as the design
progresses.
Limiting capacities for
rural and commuter roads are given in Table 4.3 of the NRA Road Needs Study
July 1998 (RNS). This document also recommends,
in section 4.9, that where dual carriageway is indicated and where traffic
projection at saturation is at 18,000AADT or less, extended traffic surveys be
undertaken to validate or amend the design traffic figures, as a basis for
determining the final road type selection.
This situation applies to the section of the route between Macroom and
Ballincollig.
Further traffic assessment
has been undertaken as part of the route selection process and the maximum
threshold for a wide single carriageway is only marginally exceeded for this
section when classed as a rural route.
At this stage, it is proposed to progress the scheme with the section
between Macroom and Ballincollig being a wide single carriageway. It is proposed that this route would be a ‘protected
road’ with limited access.
Further traffic assessment
will be undertaken to review the growth and distribution of traffic throughout
the period of one year. When funding is
made available, and construction is confirmed, the design year can be defined
with more certainty and a new assessment will be undertaken to confirm or
reject the selected cross-sections.
The further assessments
will need to establish whether traffic flows using the route are rural,
commuter or tourist. Other factors that
affect the capacity of the road are percentage of heavy good vehicles, the
gradients of the route, overtaking values and junction spacing.
The NRA are currently
investigating alternative road cross sections to cater for traffic with flows
that require a cross section between wide single carriageway and dual
carriageway. As well as assessment of
capacities, safety is another important consideration. Also, if it is found that traffic volumes
exceeding the threshold for wide single capacity fail to provide a satisfactory
level of service between Macroom and Ballincollig, then an alternative
cross-section may be considered. One
such possibility is a 3-lane carriageway with overtaking sections alternating
between opposing traffic flows. It is
envisioned that the opposing traffic would be divided by a safety barrier to
mitigate against this type of road’s questionable safety record. This 3-lane option has been used in Sweden
but is not a standard cross-section in the NRA DMRB.
Between Macroom and
Ballincollig, the initial cost of the scheme is less for wide single cross
section than dual carriageway cross-section.
However, due to the additional speed/time benefits of dual carriageway,
the economic analysis of the scheme (see Chapter 19), indicates that the
cost/benefit ratio and net present value (NPV) would favour dual carriageway
over wide single carriageway. These
benefits include: dual carriageway
would equate to level of service C whereas the wide single carriageway would
have level of service D or E (given rural classification), leading to greater
delay to drivers. Dual carriageway also
offers additional safety benefits.
Reference is made in the
Route Selection Report to 3 phases of construction:
Ballyvourney to Macroom
Macroom Bypass
Macroom to Ballincollig
Further investigation of
the economics will be carried out to find the most effective way to Phase the
works. The phasing may also affect the
cross-section as the design year is revised.