1.    Frog Hollows’ Roundabout vs. NTG / City of Darwin promoted design:...................... 3


2.    General concerns related to the Jacob’s traffic study ..................................................... 4

3.    Shortcomings of Frog Hollow Roundabout .................................................................... 7


4.    Reference Documents ..................................................................................................... 8


5.    Consultation & Engagement ........................................................................................... 9

6.    Vision on alternate strategy ............................................................................................ 9


7.    Next Steps ..................................................................................................................... 10


8.    Footnote ........................................................................................................................ 10






4th December).




Nick Hanigan (NH) is a long term resident that believe short traffic times are a key value that distinguishes Darwin in the competitive landscape of cities. Why does Darwin have such great work

/ family balance, why do we have the best participation rates in the country? One part of that

answer is traffic flows.


NH became concerned about the NTG current proposal primarily considering two key risks:


i.             First when considering the potential of the Goyder Grid becoming marginalised to 2 of 5 key streets. Mitchell & Smith surviving (Smith with a Mall through it) and Cavenagh and McMinn St hobbled, with Wood closed.

ii.            Second, noting the Palmerston draw of traffic from the Civic Core requires that (and all Tiger Brennan drive traffic) to exit the city via a right hand turn. Right hand turns are inefficient.


This document discussed the report, ‘i3 Consultants WA, Review of Jacobs Memorandum 27 May

2016 (Modelling Memo).  It is understood the Jacobs Memorandum 27th May was used by the NTG

to promote the traffic needs of Darwin’s future traffic needs.




Traffic models are used to test weaknesses in a grid system. Key statistics represent the performance of a gridThese are as follows:


·    VISSIM (a report scorecard): Like grades at school, this provides ratios and levels of service on an Australian standard. A is the best, D is showing signs of strain, E is about to collapse, F has failed. See for more information.

·    SIDDRA (test results): provides the actual performance criteria in e.g. Average Delay or

length of expected Que (95th percentile, i.e. most of the time).

·    For the terminology of this summary, I will refer to VISSIM & SIDDRA in the following way: NTG D (42.9sec // 114m) vs FHR A (5.6sec // 10.9m); means NTG model expects a level of service of D, with an average delay of 42.9sec and que length of 114m, vs. Frog Hollow Roundabout with a level of service A, average delay of 5.6sec and 10.9m que length.



·    Part of the stress testing extrapolates today’s demand to 2031. The assumptions about the

starting data set & distribution are also very important to this review.


When reviewing the traffic modelling, it is important to look beyond the average score for an intersection. Ki2 (Key Intersection 2: Barneson & Tiger Brennan) for example on an average will





always look better in the morning because the slip lane left is unobstructed (helping overall average).


Points of stress (associated with long times of waiting and to clear) are typically right hand turns (RHT’s) and these in particular should be examined in close detail.  To simplify discussions, I have focused only on the outbound traffic, but give reference for readers to find inbound stats.


As traffic stress on a roundabout increases from D to E to F, the incremental changes in time increase by say 15 seconds then 135 seconds (15 + 120 cycle). F is the worst traffic outcome, as the approaching traffic has great difficulty clearing the system. Traffic would otherwise search for other routes, but the model does not determine these routes, unless the intersections are linked. The two linked intersections in Jacob’s model is Ki1 and Ki2.


Please note, the only ‘linked’ analysis performed in this report is between Ki1 and Ki 2 (McMinn & Barneson) and (Barneson & TBD).  Flow on affects relating to the inner city traffic impacts are therefore not well understood without looking at the bigger picture.





1.                  Frog Hollows’ Roundabout vs. NTG / City of Darwin promoted design:


The Department of Infrastructure, Planning & Transport (DIPT) provided a report to City of Darwin (circa 24th Oct 17) that represented a roundabout as a bad traffic outcome because ‘all legs of the intersection would not be equal’ (ref 3.3., page 12 / 19).  Actual investigations prove otherwise:








Key Stats (see i3 traffic report page 16):


·    Overall performance improvement: NTG D (42.9sec // 114m) vs FHR A (5.6sec // 10.9m)

·    Afternoon Barneson Traffic out Town (McMinn neb right into Barneson): NTG D (53.4 sec //

106.9m) vs FHR A (9.3sec // 18.1m).





·    BRT (the high speed transit for light rail or bus provision): NTG E (68.9sec // 14.3m) vs FHR B (12.0sec vs 1.9m).

·    Through traffic, maintaining current user experience (McMinn neb straight ahead): NTG D

(53.4sec / 106.0m) vs FHR A (9.3sec / 18.1m).





In summary, this significantly improves the intersection, fixing up the public transport route and also offering the following flow on benefits:


·    Ki2 Barneson Blvd / Tiger Brennan Dr traffic out of the city is improved (TBD nwb right into

TBD): NTG D (43.7sec // 179.9m) vs FHR C (34.5sec // 162.9m).

·    This plan allows for a ‘high traffic use’ in the South East corner of the Shell site, noting that traffic leaving that site will not place load on the inner city grid when it exits. (As must turn Left and otherwise transit city vs. quickly loop the roundabout, should it need to go to places other than the city).

·    It allows City of Darwin to invest in Heritage of the Frog Hollow site, and improve Public

Open space for the future of the city’s growth plans.








2.                  General concerns related to the Jacob’s traffic study


In reviewing the Jacob’s traffic model, my concerns in reading the report are as follows:


·    Poorly reviewed: the basis of the original data set is not well understood, our feeling is the numbers are under-represented, however these assumptions are ‘black boxed’ away from the reader.  Further: inconsistent models used in the analysis and lost cars between the intersections.







·    The fundamental assumption that drives the model is 60% of traffic will use the new central spine, with 40% using the existing Bennet St leg to exit.

§    Understanding this, it says the growth region in the next 15 years will be between Woolworths & Daily St western end, in the preference of the current balance of traffic flows.

§    This is counter to the NTGs & Property City of Darwin’s stated vision of

intensification around a core, to understand this strategy:

·    If we have 20,000 people / day using our city, it is easier to create a vibrant feeling if it is in half the space rather than double the space.





§    The area ‘activated’ in the Goyder grid by this assumption is about as big as Melbourne’s CBD. They have circa 2,000,000 people using their city each day.

·    If this ratio is not correct, it places stress in the following way at Ki2 (Barneson & Tiger

Brennan Dr):






§    The level of service of (TBD nwb right into TBD) is NTG D (43.7sec / 179.9m).

In other words, this means: Acceptable delays, close to the limit of stable flow. All drivers are restricted in their ability to manoeuvre within the traffic stream. Small increases in traffic flow may cause operational problems.

·    In numbers, there are circa 1800 cars entering the city by TBD during peak hour and exiting

900 cars make this right hand turn. If the split assumption was 50:50, this number would be in the region of 1,125; if the split assumption was 40:60, this number would be 1350.

·    The additional risk in this design, is that if this fundamental base line mix is wrong, the alternate routes are to place an overlay of traffic into the city (via McMinn or Cavenagh), for which only RHTs provide the exit.
































Ref: page 7 & 28  of i3 report.


·    Ki5 (Barneson & Cavenagh St), while the model generally allows for pedestrian movements

& BRTs (transit lanes), the model does not allow them for the Cavenagh St termination. It was advised to me by DIPT that this was the intended future destination of the transit terminal.

·    Generally, the traffic movements at this new intersection appear fine, however concern about the already high number of RHT from Barneson into Cavenagh St was flagged in Jacob’s report, from report page 9:










·    While this newly introduced traffic to Cavenagh St does not as presented cause traffic delays to the traffic at this intersection, it joins the existing traffic that uses the Cavenagh St / Daily St intersections (312 cars / hour meet 677) provide a 216% increase or 677 car increase of traffic to Cavenagh St / Daily St intersection.

·    The impact of these cars on the Goyder Grid has not been modelled.

·    The actual intersection should be modelled to include the 4 bus bays shown on map & commentary on how the pedestrian movements will be handled, there is concern re. movement space for the through lane at Cavenagh St and the appropriateness of blocking off the entrance to Woolworths from that street.





·    Actual details about the New City of Darwin carpark, as it is reported in the Jacob’s report,

are obscure and it is impossible to tell if these will add further pressure to this intersection:






Ref: page 10 of Jacob’s traffic report.








3.                  Shortcomings of Frog Hollow Roundabout


The alternate solution FHR presents a more efficient traffic model at Ki1 (McMinn & Barneson), however due in some part to its success it crashes the McMinn / Daily St intersection. The comparison of the models here:




Page 10 i3 report FHR Outcome


·    McMinn nwb straight ahead: NTG C (30.1sec // 41m) vs FHR F (204.5sec // 1000m)

·    McMinn nwb right into Stuart: NTG D (36.3sec // 41.5m) vs FHR F (131.7 sec / 1000m)


To understand the magnitude of the problem, focusing on the McMinn traffic exiting the city, the

Jacob’s traffic modelling allowed for traffic as follows:




Traffic entering McMinn & Stuart Highway PM 2031:




i3 Model


Left Hand Turning




Straight Ahead




Right Hand Turning









The report analysis did not explore resolving this traffic issue as it is beyond the scope of our initial engagement. We consider the issue to address is drivers wishing to reach Gardens road or turning Right to enter Stuart for the Northern Suburbs.


[*** NH note: Jacobs did not release the baseline data and i3 Consultants pieced together the intersection data from their outputs. See footnote at end for comment on i3’s work to get to a common data set.]


We do however note that:


1.   Load balancing alternative exist for traffic users exiting the city, with:

a.    the use of Cavenagh & Daily St for straight ahead and RHT options and b.   the use of FHR with Barneson & Woolner Rd.

c.    we note, the inclusion of FHR improved exiting traffic at Ki2 (Barneson / TBD) and we expect it will here too.

2.   Local design review can also potentially offer a solution, noting up to 40% of the

intersection’s capacity allows for movements ahead or LHT. Therefore:

a.    Allowing only local traffic (i.e. no through connection to Gardens Road) ahead and b.   Removing the LHTing lane may also improve this result.


To close, the extra traffic introduced into this system is 323 cars (in PM peak).  Further work needs to be done to review the impact in the context of an integrated grid system.








4.                  Reference Documents


Here are the documents that are the backbone of the discussion:


·    Barneson Boulevard and Tiger Brennan Drive Stage 3 – Report to City of Darwin (Oct 2017)

·    Jacobs Memo – Traffic Modell Assessment (May 2016)

·    i3Consultants – Alternative Model Review (Nov 2017)

·    Barneson Blvd website: search Barneson Blvd.





5.                  Consultation & Engagement


The consultation and engagement seemed to intentionally hold back the details of the Schematic Design & Traffic Report. The Schematic Design is not on the project website, the traffic report required 2 letters to the Minister & some support from Property Council of Australia (NT) to release it to the public.  This feedback appears late in the consultation cycle but we have only been able to review this from early Oct and we waited until 24th October DIPT before proceeding.  I believe we have provided this latest feedback swiftly as there is plainly urgency in this project.


In review of this project, I believe:


·    It is not unreasonable to ask and expect the detailed traffic report for the project & city to be released. This is where my initial concern started and I believe the modelling presented does not fully resolve this (note: recommend need for sensitivities on 60:40 assumption and understanding knock on affects within grid e.g. Cavenagh & Daily St).

·    The City of Darwin’s car park plans and intention to develop their land has clearly been a driver in the background.  I do not believe their interests were clearly enough expressed in the consultation process and in particular notification to the public on:

a)   How they would manage their conflict of interest (in other states independent review panels are set up).

b)   What / Why their development direction is better than the alternative.

·    I have been advised, the original plan for Barneson was underground, the next plan was to finish at Barneson. The alternative plan against which the City of Darwin should measure their Development is to stop at Barneson.


The City of Darwin are responsible for protecting thevalues’ of the city. This project has been a litmus test for ‘commuter times’ (read: ability to get home to family or ability for Darwin to maintain the strongest sports participation in the country), heritage (read: ability to create a distinctive identity) and public open space (read: ability to provide a tropical environment in a urban metropolis).





6.                  Vision on alternate strategy


The merits of stopping at McMinn:


·    Recommend to spend $5m on the arrival statement (rather than $10,000 on painting Woolworth’s wall). The statement can reflect to the entrants something about the value of Darwin – improving Darwin’s Public Open space account.

·    Recommend that the City of Darwin finds a way to facilitate CDU to use the Old Darwin School, as a way to activate the park create a new layer of heritage – improving Darwin’s heritage values.





·    Recommend that the traffic modelling be reviewed with more rigor, particularly the sensitives raised above.





7.                  Next Steps


I recommend as next steps:


·    Continue engagement to fully explore the traffic implications as outlined in the report attached.

·    i3 Consultants WA have worked with the City of Darwin before and charge $195/hr.

·    I am available to assist in review of any documents should you so wish to reach out to me to discuss.





8.                  Footnote


How i3 standardised the data set, copied form email seeking explaination:






Hi Nick – this is the messy part of the whole process.


The data in my model is the same as the data in the Jacobs VISSIM model – the differences are in the analysis:


Jacobs allows right turns from Daily into McMinn in their VISSIM model but does not include a SIDRA model that shows how this would be accommodated;

Without the SIDRA model, I could only model this intersection as per the current layout and hence I simply ignored these turns. It would be erroneous to assign these turns to another movement (e.g. straight through) as it is likely that these right turners would simply turn right into Woods or Cavenagh St from Daily St and hence would not travel through the

intersection at all.


Below is an explanation for how I re-assigned traffic with the alternate layout. It’s fairly

simplistic but it has to be given the constraints.





Figure 1 – AM Peak Hour re-assignment





Figure 2 – PM peak hour re-assignment



Kind regards, David