Porirua City Council Transmission Gully Options - 12 August 2005
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Porirua City Council Transmission Gully Options 12 August 2005
Porirua City Council Transmission Gully options
1. Introduction
This report provides information for the Regional Land Transport Committee
for a workshop on 18 August 2005.
Porirua City Council has proposed a number of options to provide an
affordable Transmission Gully option. These options include a two lane option
for the full length, a two lane reversible flow option, a two lane option with
auxiliary lanes and a northern four lane option from MacKays Crossing to
SH58. The latter option is proposed in conjunction with improvements along
SH58 and SH2. We have reviewed these options and report as follows.
Recently Transit New Zealand has arranged a peer review of the cost estimates
of TGM and the Coastal Route. As a result of the discussions Maunsell has
revised the cost estimates. The final expected cost estimate comparison is
shown in the following table. The differences between the estimates relate
largely to differences in scope and extent of mitigation.
Element Maunsell Peer Maunsell
original review revised
expected expected expected
cost estimate cost cost
estimate estimate
Coastal Route $610M $890M $700M
Transmission Gully Motorway $1,001M $1,170 $1,090M
The Maunsell revised estimates are used in this report. Assessed estimates have
been adjusted in relation to the change in the estimate for the full TGM.
2. Two lane Transmission Gully option
A two lane Transmission Gully element was considered during the initial
element assessment in the WCTS – Draft technical report, April 2005. The
Wellington Transport Strategy Model (WTSM) did not discriminate between a
four lane version and a two lane version. The initial results were reported but
the following note was added based on engineering judgement.
“ A two lane road would be impractical on the steep grades and may not have
the safety features associated with a four lane expressway. The model benefit
calculations do not sufficiently account for the resultant merges or the gradient
effects”
The element was not considered in the scenarios on this basis. In response to
Porirua City Council we have given further consideration to a two lane option.
a. Gradients and vehicle speeds
The average speed on a two lane Transmission Gully would be 70kph.
Due to the steep grades Transmission Gully comprises a series of grades that
are back to back.
The following table indicates the gradients involved on the Transmission Gully
route in the northbound direction.Location Gradient Length
(km)
Linden 4.7% 1.2
3.4% 1.6
Warspite Ave -4% 1.5
-1.4% 1.8
James Cook 1.5% 0.9
Interchange
SH58 -7.7% 1.8
3.6% 1.7
0% 1.0
1.5% 2.5
-3.7% 1.1
2.5% 3.9
Wainui Saddle 5.5% 2.0
MacKays Crossing -8% 3.3
Preliminary calculations indicate that average truck speeds with a 5kW/tonne
power to weight ratio will be 60 kph northbound and 69 kph southbound.
Austroads “Rural Road Design” table 2.1 identifies that the 85% speed in
mountainous country is 70kph.
The Road Transport Federation identifies that heavy vehicles typically descend
steep grades in one gear higher than they ascend them which would equate to a
speed equal the uphill speed plus 10%.
Current truck speed/ gradient curves in the TNZ Geometric Design Manual
indicate that truck speeds on the steeper 8% gradients will drop to 20 kph. A
two lane median divided road in this scenario would perform very poorly; the
slowest vehicles would dictate the speed of the road.
b. Regional benefits, projected volumes and levels of service
The Wellington Transport Strategy Model (WTSM) was run with a Reliability
style package for the Western Corridor. The only difference being the choice of
route through the central section of the study area; coastal route (CR), TGM
and a two lane TG (TG2) based on a 70kph average speed.
The key results are shown in the attached table. It is noted that a two lane full
length TG route will only remove 10% of traffic along Mana Esplanade from
the base case and has less regional benefits than the Coastal Route.
Reliability Regional Transmission Gully Mana Esplanade
Package Benefits
Daily Level of Daily traffic Level of
traffic service volume service
volume (AM Peak) (AM Peak)
Base Case - - - 30,440 E
Coastal Route $50.0M - - 13,830 Note 1 C
Transmission $52.1M 26,290 B 16,880 C
Gully Motorway
Transmission $43.2M 12,600 C Note 2 27,400 E
Gully
Two lanesNotes to table
1. The traffic volume on the Mana Bypass would be 23,190 vehicles per day.
2. The level of service is based on a volume capacity ratio. Given the steep and
numerous grades and lack of passing opportunities on TG2 it is expected that
users will perceive a lower level of service.
3. The levels of service can be briefly described as
A – Free flow
B – Good flow
C – Satisfactory
D – Minor Delays
E – Major Delays
F – Total flow breakdown
4. The base network assumes Mana Esplanade Clearways add 20% to current
capacity and are removed under the options
5. Benefits exclude crash savings and intangible benefits
c. Project efficiency
A two lane TG route is less efficient than either a four lane TG route or the
Coastal route. The amount of the regional benefits for the Optimised
Reliability package without road improvements to the central section is
$33.4M, deducting this sum from the package benefits provides the relative
benefits of the elements. A relative project efficiency figure can then be
calculated by comparing the modelled regional benefits divided by the costs as
shown in the following table.
Package Regional Expected Cost Relative
Benefits for efficiency
Central Section between
elements
Coastal Route $16.6M $700M 2.0
Transmission Gully $18.7M $1090M 1.5
Motorway
Transmission Gully $9.8M $830M 1.0
Two lanes
d. Planning balance sheet
A two lane TG route has a lower planning balance sheet score than either the
Coastal Route or the four lane TGM in a reliability style package.
The planning balance sheet scores have been updated for these packages as
shown in the following table.
Reliability Planning Balance Sheet Score
Package RLTC LTNZ TAG T2000+ Public
Weighting
Note 1
Coastal Route 6.2 6.4 6.3 6.4 6.4
Transmission Gully 5.9 6.5 6.1 6.5 6.2
Motorway
Transmission Gully 5.8 6.2 5.9 6.2 6.0
Two lanes
Note 1. The Public Weighting was derived from the Phase 2 Consultatione. Safety
The gradients on TG2 would create large differential speeds between vehicles
resulting in a higher crash rate. Typically cars have sufficient power to
continue at 100kph on most grades while heavily laden vehicles and less
confident drivers will slow. The speed differential will occur on both uphill and
downhill grades. The speed differential creates a hazard which would result in
a higher crash rate. Transit New Zealand has advised that they would not
construct a two lane road on this route.
f. Driver frustration
The limited opportunities to overtake, the steep gradients and the total climb
height of the TG route would result in a large amount of frustration for
motorists. For instance a truck crawling up the 3.3km long climb to Wainui
Saddle would create frustration for the queue of vehicles caught behind.
g. Reliability
A two lane road is less reliable than a four lane road. The additional width of
four lane roads allows the traffic stream to pass partial blockages arising from
breakdowns and crashes that would close a two lane road, particularly if it is
median divided. Note this effect would be offset by the increased reliability of
two roads if the roads were coupled with an ATMS system to warn
approaching drivers of closures.
h. Summary
A two lane version of Transmission Gully would have an average speed in the
order of 70kph with a wide range of actual vehicle speeds resulting in driver
frustration and increased road crashes.
It would have a lower PBS score and project efficiency than either a four lane
TGM or the Coastal Route. It would not reduce traffic volumes on Mana
Esplanade significantly.
We do not recommend that it is considered further.
3. Two lane Transmission Gully with reversible flow
A two lane Transmission Gully was considered in the Draft technical report.
The modelling identified that the regional benefits for this option were $7.2M.
There are significant counterpeak flows that do not easily fit a one way road. In
the morning two hour peak period the four lane TGM carries 2,710 vehicles
southbound and 1,550 vehicles northbound. Interpeak the volumes are
approximately equal. The approximate 60/40 split does not suit a one way
reversible road, 40% of traffic that could use TGM are unable to use it.
It would have a relative efficiency less than a two lane TG. Given the long
length of TGM and the number of intersections a considerable ‘down-time’
during the change over period would be required further reducing its
efficiency.4. Two lane Transmission Gully option with auxiliary lanes
The cost of a two lane road with appropriate passing lanes would be in the
order of 95-99% of the four lane version. If the passing lane version
successfully attracts similar traffic volumes to the four lane version the merges
at the end of each passing lane could generate delays and disruption.
A two lane Transmission Gully route with correctly designed auxiliary lanes
would score similarly to a four lane version but would have a lower
performance. Reducing the lengths of auxiliary would decrease the road
performance and PBS assessment commensurately.
The Transit New Zealand Geometric Design Manual recommends climbing
lanes where the grades cause a speed differential of 15kph between heavy
vehicles and normal vehicles. The manual also notes that steep downgrades
should also be considered. The required length of auxiliary lanes on the TG
route using the Geometric Design Manual criterion would be in the order of
29km.
The auxiliary lanes would be required in the more expensive sections on the
steep grades and two lane sections would be typically on the less expensive
sections on the flatter grades. For example the expensive viaduct sections on
the 8% grade between MacKays Crossing and the Wainui Saddle would need
to be four lanes.
The small cost reduction from the four lane version is likely to be offset by a
similar reduction in regional benefits. If the passing lane version successfully
attracts similar traffic volumes to the four lane version the merges at the end of
each passing lane would have an increased accident rate and could generate
delays and disruption depending on the number of vehicles using TGM.
The passing lane version is likely to score somewhere between the two lane
version and the four lane version. A Transmission Gully route with passing
lanes would be inferior to a four lane option and any potential savings in cost
would be matched by a consequent reduction in PBS score, facility and
national and regional benefit.
5. Four lane Transmission Gully – MacKays Crossing to SH58
A MacKays Crossing to SH58 four lane transmission gully option was
considered in the WCTS – Draft technical report. The revised expected cost for
this option is $720M with $8.9M regional benefits.
Porirua City Council has proposed an option that also includes improvements
along SH58 and SH2 and reductions in speed on SH1 to complement the
option. The cost of the proposed improvements along SH58 and SH2 has not
been estimated but it is likely it would be in the order of hundreds of millions
of dollars.The northern section of Transmission Gully (4 lanes) along with the SH2 and
SH58 improvements would attract 16,000 vehicles per day and would reduce
predicted flows in Mana by 15%. A further 11,000 vehicles per day would
transfer to the northern section of TGM if SH1 is downgraded by traffic
calming measures, although this increases the cost of their journey and would
result in a significant increase in traffic on SH58 and SH2.
The projected traffic volumes along TGM and Mana Esplanade for these
options are shown in the following table.
Package Transmission SH1 (Mana)
Gully
Base Case - 30,440
Transmission Gully Motorway North with 16,030 26,110
SH2 and SH58 improvements
Transmission Gully Motorway North with 27,010 15,310
SH2 and SH58 improvements and SH1 traffic
calming
Transmission Gully - two lanes 12,600 27,400
Transmission Gully Motorway – four lanes 26,290 16,880
The relative efficiency of the four lane Transmission Gully north options can
be compared to the other options as shown in the following table. The model
identifies that the SH2 and SH58 improvements provide $28.1M benefits
without any Western Corridor projects. Without the SH2 and SH58
improvements the four lane northern TGM is less efficient than the two lane
option. With SH2 and SH58 improvements the option has a similar efficiency
to a two lane version of TG. Both versions are less efficient than either the full
TGM or the Coastal Route.
Package Regional Expected Cost Relative
Benefits for of central efficiency
Central Section section between
elements
Coastal Route $16.6M $700M 2.0
Transmission Gully $18.7M $1090M 1.5
Motorway
Transmission Gully $9.1M $720M 1.1
Motorway North with SH2
and SH58 improvements
Transmission Gully $5.7M $720M 0.7
Motorway North without
SH2 and SH58
improvements
Transmission Gully $9.8M $830M 1.0
Two lanes
It is noted that diverting Kapiti- Wellington traffic via TG/ Hutt Valley would
add approximately 6km to the journey length. It would also add vehicles to the
congestion on SH2. Similarly diverting Kapiti- Porirua traffic via TG would
add approximately 5km to the journey length and would increase traffic
volumes substantially around the Pauatahanui Inlet. In contrast, Kapiti/ HuttValley traffic benefit by a northern TG option with a saving of approximately
7.5km from the current SH1/SH58 route.
Without the SH2/SH58 improvements a northern TG option delivers fewer
benefits than any of the other options. With the improvements it would deliver
similar project efficiency to a two lane TG. There appears to be a greater
project efficiency to complete the southern section of TG rather than make the
proposed improvements along SH2 and SH58.
6. Summary
The options put forward by Porirua City Council provide fewer benefits than
either a full TGM or a Coastal Route.
A four lane TGM is the optimal highway configuration on the Transmission
Gully route primarily because of the steep grades. A four lane configuration
has been previously identified through the long history of attempting to fund
this route and was the preferred scheme for this route prior to this study.
Construction of the northern section of TGM only provides few benefits
because it does not link directly to a north/ south route thereby requiring all
traffic to travel east/ west before continuing their north/ south journey.
Combining the northern section with improvements away from the corridor
would likely be of similar cost to the full TGM and would have less project
efficiency than TGM.
The options proposed by Porirua City Council do provide a method to build
along part of the Transmission Gully route for a similar cost to the Coastal
Route. However unlike the Coastal Route option they do not provide a
complete solution for the Central Section (MacKays Crossing to Linden) and
would only be seen as an interim stage of a full TGM.You can also read
