The Route Options under
consideration have been designed in accordance with the requirements of the NRA
Design Manual for Roads and Bridges.
The route options to the
east of Macroom have been considered for both dual and single carriageway
alignments. The assessment contained in
this chapter is based on the single carriageway alignment. See chapter 21 for details of the
cross-section of the road.
Under the instruction of
the NRA, a Design Speed of 100km/h has been used for the standard single
carriageway and wide single carriageway sections of the scheme. A design speed of 120km/h has been used for
dual carriageway sections. An extract
from Table 4, NRA DMRB TD 9/01 ‘Highway Link Design’, highlighting the
characteristics of the road types proposed on the scheme, is shown below:
Table 16.1: Extract from NRA DMRB TD 9/00; Table 4:
Recommended Rural Layouts
|
Category |
Type of Road |
Capacity (AADT)* |
Edge Treatment |
Access Treatment |
Minor Road Treatment |
Major Junction Treatment |
|
2/3A |
Standard
Single (7.3m) Carriageway S2 |
11,600 |
2.5m
hard shoulders |
Minimise
number of accesses to avoid standing vehicles and concentrate turning
movements |
Priority
junctions, with ghost islands where necessary |
Ghost
islands or roundabouts. |
|
3B/4 |
Wide
Single (10m) Carriageway WS2 |
13,800 |
2.5m
hard shoulders. Pedestrian usage
minimised. |
Minimise
number of accesses to avoid standing vehicles and concentrate turning
movements |
Ghost
islands. Some side roads stopped
up. Occasional bridges at higher end
of traffic range |
At-grade
roundabouts |
|
5 |
Standard
At Grade Dual2 Lane (7.0m) Carriageways. All Purpose D2AP |
26,500 |
2.5m
hard shoulders |
Minimise
number of accesses to avoid standing vehicles and concentrate turning
movements No gaps in the central reserve. |
Priority
junctions. No other gaps in the central
reserve |
At-grade
roundabouts. Grade separation if economically justified. |
|
5A |
Standard
Grade Separated 2 Lane (7.0m)Dual Carriageways. All Purpose D2AP |
42,000 |
2.5m
hard shoulders |
No
access. |
Left
in/Left out only. No gaps in the central
reserve |
Full
grade separation |
*
Capacity (AADT) for Level of
Service D with level terrain
All the Route Options on
the scheme tie in with the upgraded N22 west of Ballyvourney and the proposed Ballincollig
Bypass at the Barnagore footbridge west of Ballincollig.
The geometric design of
the route options conforms to the standards in the NRA DMRB. The geometric design such as horizontal and
vertical alignment, overtaking values and stopping sight distances is governed
by NRA TD 9/01.
The geometric design of
the route options at this stage is conceptual in nature and allows a
comparative analysis of the route options.
Based on more detailed information at successive stages of the scheme,
full optimisation of the alignments will be undertaken for the selected
Preferred Route.
Possible Relaxations or
Departures to the design parameters become necessary in order to optimise cost/benefit
and environmental mitigation. No
departures from the design standards are anticipated at this stage.
The basic principles used
in the development of the route alignments are as follows:
For single carriageway
design, the use of overtaking and non-overtaking sections to give a clear
perception of safe overtaking opportunity to drivers is utilised. Band D (curves of radius between 510m and
1020m) horizontal curves combined with non-overtaking crests, sag curves and
gradients make up the non-overtaking sections, while Full Overtaking Sight
Distance (FOSD) crests, sag curves and gradients make up the overtaking
sections. Junctions have been located
at non-overtaking sections to improve safety.
Assessment of the best means to achieve satisfactory overtaking will be
undertaken at preliminary design.
Overtaking can be achieved by a combination of horizontal and vertical
alignment, climbing lanes and junction strategy.
For dual carriageway
design, short curves and straights have been eliminated to give smooth flowing
alignments.
Due to the difficult
nature of the topography in the area and the high level of environmental
constraints, there are a number of relaxations in horizontal and vertical
curvature to reduce costs and visual impact, predominantly by reducing
earthwork. The relaxations in
horizontal geometry on the single carriageways serve a twofold purpose, the
avoidance of constraints and the definition of clear non-overtaking sections.
In the case of the vertical curvature relaxations, it is necessary to reduce
vertical crest curves in non-overtaking sections to one step below desirable
minimum, in order to avoid dubious overtaking opportunities.
According to the
guidelines in NRA DMRB TD 9/01, climbing lanes will be considered where
vertical gradients exceed 2% for longer than 500m. On standard and wide single carriageways, the climbing lane
should replace the hard shoulder, resulting in small additional width. Due to
the fact that the cost of provision of the climbing lane is small, climbing
lanes will be considered where the risen height exceeds 15m, or wherever their
use would be of advantage in allowing slow moving climbing traffic to be
overtaken.
An extract from Table 3,
NRA TD 9/00, detailing the required geometric design criteria for each of the
three road types in the scheme, is shown in Table 16.2 overleaf.
Table 16.2: extract from NRA TD 9/00; Table 3:
Required Geometric Design criteria
|
Cross
Section |
5 |
3B/4 |
2/3A |
|
Design
Speed |
120km/h |
100km/h |
100km/h |
|
STOPPING
SIGHT DISTANCE (m) |
|
|
|
|
Desirable Minimum |
295 |
215 |
215 |
|
1 Step below Desirable Minimum |
215 |
160 |
160 |
|
2 Steps below Desirable Minimum |
160 |
120 |
120 |
|
HORIZONTAL
CURVATURE (m) |
|
|
|
|
Minimum R without elimination of Adverse Camber &
Transitions |
2880 |
2040 |
2040 |
|
Minimum R with Superelevation of 2.5% |
2040 |
1440 |
1440 |
|
Minimum R with Superelevation of 3.5% |
1440 |
1020 |
1020 |
|
Desirable Minimum R with Superelevation of 5% |
1020 |
720 |
720 |
|
1 Step below Desirable Minimum R with Superelevation of
7% |
720 |
510 |
510 |
|
2 Steps below Desirable Minimum R with Superelevation of
7% |
510 |
360 |
360 |
|
VERTICAL
CURVATURE - CREST |
|
|
|
|
Desirable Minimum Crest K Value |
182 |
100 |
100 |
|
1 Step below Desirable Minimum Crest K Value |
100 |
55 |
55 |
|
2 Steps below Desirable Minimum Crest K Value |
55 |
30 |
30 |
|
VERTICAL
CURVATURE - SAG |
|
|
|
|
Desirable Minimum Sag K Value |
53 |
37 |
37 |
|
1 Step below Desirable Minimum Sag K Value |
37 |
26 |
26 |
|
2 Steps below Desirable Minimum Sag K Value |
26 |
20 |
20 |
|
OVERTAKING
SIGHT DISTANCES (m) |
|
|
|
|
Full Overtaking Sight Distance FOSD |
N/A |
580 |
580 |
|
FOSD Overtaking Crest K Value |
N/A |
400 |
400 |
The following section
attempts to describe briefly each section of the route options under the
headings of horizontal alignment and vertical alignment.
The alignment is limited
by the tie-in to the existing N22 to the west of Ballyvourney and the need to
avoid properties and limit cut to the north.
This is a short section with no overtaking opportunities. Relaxations may be required for crest curve
to prevent excessive cut. 2 junctions.
The alignment is limited
to the north of Ballyvourney to avoid properties and limit cut to the
north. This is a short section with no
overtaking opportunities.
Relaxation to crest curves
may be required to prevent excessive cut and prevent dubious overtaking
sections. Good overtaking opportunities
with long overtaking section approx. 3km.
2 junctions.
Relaxation to crest curves
may be required to prevent excessive cut and prevent dubious overtaking
sections. Relaxations may also be
required to the horizontal alignment to avoid constraints. There is minimal overtaking opportunity in
this section, as there are 5 junctions in this 5km stretch of road.
Good overtaking
opportunities. Relaxation to crest
curve may be required to prevent excessive cut and prevent dubious overtaking
section. One junction located to the
west of the section.
Relaxation to crest curve
may be required to prevent excessive cut and prevent dubious overtaking
section. There are two proposed
junctions to the west of this section with minimal overtaking opportunity.
Relaxation to crest curve
may be required to prevent excessive cut and prevent dubious overtaking
section. Horizontal relaxations may
also be required to avoid constraints.
Good overtaking opportunities in several lengths. There are seven proposed junctions.
Alignment is limited by
the constraints and the need to cross over the existing N22 at the Ballincollig
bypass. Horizontal and vertical
relaxations will be required and there are no overtaking opportunities.
The alignment is limited
by the tie-in to the existing N22 to the west of Ballyvourney and the need to
avoid properties and limit cut to the north.
This is a short section with no overtaking opportunities. Relaxations
may be required for crest curve to prevent excessive cut. There is one proposed junction.
The alignment is limited
to the north of Ballyvourney to avoid properties and limit cut to the
north. This is a short section with no
overtaking opportunities.
The alignment is limited
to the north of Ballyvourney and Ballymakeery to avoid properties and limit cut
to the north. Vertical and horizontal
relaxations will be required. There is
an overtaking length between the junctions. There are 5 proposed junctions.
Relaxation to crest curve
may be required to prevent excessive cut and prevent dubious overtaking
section. Horizontal relaxations may
also be required to avoid constraints.
There are a number of overtaking lengths within this section. There are eight proposed junctions within
this section.
Relaxation to crest curve
may be required to prevent excessive cut and prevent dubious overtaking
section. Horizontal relaxations may
also be required to avoid constraints.
There is one overtaking length.
The alignment is limited
to the north of Macroom by the topography of the area. Relaxation to crest curve may be required to
prevent excessive cut and prevent dubious overtaking section. Horizontal relaxations may also be required
to avoid constraints. There is one
junction proposed to the west of this section with no overtaking opportunities.
The alignment is limited
to the north east of Macroom by the topography and development of the
area. Relaxation to crest curve may be
required to prevent excessive cut and prevent dubious overtaking section. Horizontal relaxations may also be required
to avoid constraints. There are two
junctions proposed in this section with opportunity for overtaking between
them.
This is a short section
with no relaxations necessary and one proposed junction.
There are no proposed junctions
in this section. The alignment is
limited by the constraints and topography of the area. Relaxations may not be required but
overtaking opportunity may be limited to one length.
There is one proposed
junction in this section. No
relaxations are proposed. Includes long
(1km) overtaking section.
This section of the
alignment is over flat terrain with few constraints. No relaxations are proposed and there is opportunity for several
overtaking sections.
The alignment through this
section is restricted by existing development.
No relaxations are proposed.
The alignment is limited
by existing development and other constraints at the tie-in to the Ballincollig
bypass.
The alignment is limited
by the topography and other constraints in this area. No Relaxations are proposed.
The alignment is limited
by the topography and other constraints in this area. No Relaxations are proposed.
There are overtaking sections and one major junction at intersection
with R619.
The alignment is limited
by the topography and other constraints in this area. No Relaxations are proposed.
Overtaking opportunities.
The alignment is limited
by the topography and other constraints in this area. No relaxations are proposed.
Good overtaking opportunities
This is a short section
with no relaxations proposed.
This section of the
alignment is over flat terrain. There
are good overtaking opportunities. A
junction is proposed at the intersection with the R619.
The alignment through this
section is over fairly flat terrain but is restricted by existing
development. No relaxations are
proposed and there are good overtaking opportunities.
The alignment is limited
by the topography and other constraints in this area. There are three proposed junctions with two overtaking
lengths. Relaxation to crest curve may
be required to prevent excessive cut and prevent dubious overtaking
section.
The alignment is limited
by the topography and other constraints in this area. There is one proposed junction with the R619. No relaxations are proposed.
The alignment is limited
by the topography and other constraints in this area. There are no proposed junctions.
No relaxations are proposed.
This short section of
alignment has no relaxations or junctions proposed.
An extract from Table 3,
NRA TD 27/00, detailing the cross-sectional arrangement of the three road types
in the scheme, is shown below. The standard cross sections to be used on the
scheme are detailed on Figure 16.2.and in Table 16-3
below:
Table 16-3: Extract from NRA DMRB TD 27/00; Table
3:Standard Cross-Sections for the N22 Scheme
|
|
Nearside |
|
Offside |
|
||||
|
Road
Type |
Verge |
Hard
Shoulder |
Carriageway |
Central
Reserve |
Carriageway |
Hard
Shoulder |
Verge |
Total
Width |
|
2/3A |
3.00 |
2.50 |
3.65 |
0.00 |
3.65 |
2.50 |
3.00 |
18.30 |
|
3B/4 |
3.00 |
2.50 |
5.00 |
0.00 |
5.00 |
2.50 |
3.00 |
21.00 |
|
5A |
2.00 |
2.50 |
7.00 |
3.00 |
7.00 |
2.50 |
2.00 |
26.00 |
(Details taken: The
headroom at all structures shall be in accordance with TD 27/00 of the NRA DMRB,
This requires standard headroom of 5.30m throughout the scheme.
At this stage in the
design process, the junction proposals are at a conceptual layout stage, and
are subject to change during the optimisation of the preferred route during the
preliminary design phase of the scheme development.
Junctions have been
designed in accordance with NRA TD 16/93 ‘Geometric Design of Roundabouts’, TD
42/95 ‘Geometric Design of Major/Minor Priority Junctions’, TD 41/95 ‘Vehicular
Access to All-Purpose Roads’ and TD 40/94 ‘Layout of Compact Grade Separated
Junctions’. The junction provision on
the standard single carriageway west of Macroom and wide single carriageway for
the Macroom Bypass, is priority junctions for county roads and roundabouts for
the regional roads. The wide single
carriageway east of Macroom has been design with a limited number of
junctions. It is envisioned that direct
access from properties would generally be eliminated for the entire route.
The possible junction
locations, at this stage of the development, are indicated on Figure 16.2.
Where the local road
network interfaces with the proposed road, at junctions and bridges, the
following standards have been used to assess any realignment necessary:
·
7.30m carriageway
with 2.50m hard shoulders and alignments compatible with 70kph or 85kph Design
Speeds (depending on the existing geometry) for regional roads, and
·
7.00m carriageway
with 0.50m hard strips and alignments compatible with 60kph or 50kph Design
Speeds (depending on the existing geometry).
Side road design standards
will be subject to confirmation from NRA and Cork CC for preliminary design.
Road closures have been
identified in order to minimise access onto single carriageway sections and
reduce the number of structures required on dual carriageway sections. These have been assessed in terms of detour
lengths, minimising impact on properties and constraints and the provision of
alternative access roads. The provision
of over and underbridges will also be subject to economic feasibility. The road closures on the preferred route
option will be examined in detail during the preliminary design phase.
A Stage F Phase 1 Safety
Audit has been undertaken on the all the route options. This audit reviewed each route link by link
and made comments on the present alignment and possible junction proposals that
should be considered if the route was selected as the preferred option. The audit was undertaken independently of the
design team.
All routes have been
developed to provide viable alignments given the current design stage. Revision to the alignment may be used at the
next stage (preliminary design) to reduce the impact on environmental issues following
further investigation of the route corridor.
This may involve realignment of the vertical and horizontal to minimise
impact on constraints and the landscape.
As the alignment is subject to change, the following is a comment on the
potential of the routes rather than specific geometric comparison.
The alignment is limited
at the tie-in to the existing N22 where the corridor available is limited by
the townland of Ballyvourney, which has steep hills to the north, and the
Sullane River and St Gobnaits Wood to the south. The Yellow route tie-in is further to the east than the Green and
Green/Link/Yellow routes, which will require that the existing N22 is upgraded
between the tie-in and the existing wide single carriageway to the west. The Yellow route requires a sharp change in
direction at the tie-in, which would create a delay to through traffic. All the routes pass though hilly terrain,
which, for economic design, may lead to the use of relaxations to the vertical
crest curves. Relaxations to the
vertical crest curve are also used to eliminate dubious overtaking
sections. The Yellow and Green routes
have more junctions along their routes, as these pass through areas of greater
population density than the Green/Link/Yellow route, which may have an affect
on safety and overtaking. The Green
route has fewer junctions than the Yellow route.
The routes in order of
preference for alignment in terms of safety, overtaking and relaxations to
standard are:
· Green/Link/Yellow (most preferred)
· Green
· Yellow (least preferred)
The northern routes are
limited by the hilly terrain along their length and by the need to cross over
the existing N22 at the Ballincollig bypass.
Relaxations to the vertical and horizontal design will be required to accommodate
the topography, that is reducing the cut, which is good for both economic and
aesthetic reasons. Relaxations to the
vertical crest curve are also used to eliminate dubious overtaking sections.
The southern routes pass
through relatively flat terrain once the alignment is south of Macroom. There are limitations at the tie-in to the
Ballincollig bypass where the corridor is restricted by existing property and
other constraints.
Due to the design
development of the various routes, the Green route has significantly more
junctions proposed. The other routes
have been developed with limited junctions for the section between Macroom and
Ballincollig. The issue of junctions
will be resolved at preliminary design stage and has therefore not been taken
into account of the comparison of the corridors at this stage.
The Yellow, Blue and
Yellow/Link/Blue routes are preferred as they pass through much flatter terrain
than the Green, Red and Green/Link/Red routes.
The hillier terrain of the northern routes leads to the development of
an alignment with steep gradients that will reduce traffic speed and limit
overtaking lengths.