Ask Your Mates Open Forum

Single Lane Bridge Width

  • 1.  Single Lane Bridge Width

    Posted 28 April 2014 21:20
    The Australian Standard Bridge Code AS 5100.1 recommends that for traffic volumes less than 150 vpd that the width of a single lane bridge should be in the range 4.2m to 4.5m.  

    Does anyone have any thoughts on why a council, to save costs, shouldn't specify the minimum width of 4.2m for a 26m long bridge on a sealed rural road (approx 6m pavement width)?

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    Phil Gee
    Director
    Sugden & Gee Pty Ltd
    LAUDERDALE TAS
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  • 2.  RE:Single Lane Bridge Width

    Posted 29 April 2014 23:09
    Phil

    You need to take risk management into consideration.  Does the cost saving get out weighed by possible public liability claims if someone runs into the railing or off the bridge.  The approach  roadway (carriageway) should always match the bridge roadway. 

    Whilst at Bathurst City Council I was tasked with widening culverts on Gilmore St and Sofala Rd (MR52) as part of Council's SIM contract with the RTA as the culvert roadway was less than the trafficable roadway of the adjoining road.

    If the bridge is only 26m long, is it really worth the risk?  Especially as it is new and regardless of the Australian Standard which is essentially an absolute minimum.

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    Ian Dollery
    Shoalhaven City Council
    NOWRA NSW
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  • 3.  RE:Single Lane Bridge Width

    Posted 29 April 2014 23:09
    Phil

    The only situations I can think  of where thought may be give to providing additional width would be those where there is one or a combination of the following factors; long term Heavy Vehicle Traffic component; limited bridge approach visibility; high approach speed potential; significant curvature on either the structure or approaches and or significant crossfall.

    In the absence of these issues, minimum width would be fine provided approach warning signs indicating single lane operation and give way are provided.

    Cheers
    Bruce

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    Bruce Douglas
    Sholcons Consulting
    Balnarring Beach VIC
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  • 4.  RE:Single Lane Bridge Width

    Posted 30 April 2014 22:45
    Greetings,

    Has anyone considered controlling the approaches and the use of the one-lane bridge with traffic signals - similar to mobile ones used for roadworks traffic control where only one lane is available?  These could be solar powered units (if power is not available) with radio synchronization between the two units.  Adequate signage would be needed as well as default instructions during black outs.  There are "soft options" if widening the bridge is cost prohibitive.  You might require an experienced Traffic Engineer to assist you in designing the traffic control system.  I hope these thought can assist you progress your considerations.

    Regards
    Peter

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    Peter HARPER
    Town of Gawler
    GAWLER SAau
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  • 5.  RE:Single Lane Bridge Width

    Posted 01 May 2014 00:51
    Hi all,

    AS1742.2 (Clause 4.6.2) has some information about narrow bridges as I have been looking at some recently.
    The standard includes warrants for signal control.

    In light of AS 5100.1 quoted by Phil, it is interesting that AS1742.2 says the MAXIMUM width should be 4.0 m

    The big issues, in the cases I have looked at, are actually the queuing and sight distance.
    I discussed calculating queue lengths with SIDRA Solutions, and aaSIDRA  has a method for checking  this.
    There are also some research papers for calculating queue length ~ if you are happy to get your hands dirty with stochastic processes!!

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    Hugh Dixon
    Traffic Engineer
    HDS Australia
    Glen Waverley VIC
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  • 6.  RE:Single Lane Bridge Width

    Posted 02 May 2014 23:53

    Thanks for the comments guys.  The existing bridge is single lane but is 5.4m which looks way too wide and would tempt drivers to pass other cars on the bridge.  The existing signage is designed properly to warn drivers of the one lane bridge and the current volumes would not warrant signals (I was there for an hour one morning and only about 3 cars crossed the bridge).  It was suggested to me by a bridge contractor that it should be wide enough to cater for the larger harvesters.

    It is interesting that it appears AS 5100 as AS 1742 have different recommendations on this.
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    Phil Gee
    Director
    Sugden & Gee Pty Ltd
    LAUDERDALE TAS
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  • 7.  RE:Single Lane Bridge Width

    Posted 04 May 2014 17:51
    Hi From my past experience working in Highland Region, Scotland with single-track bridges, so long as traffic volume is low, then key factors are: Provide good waiting space each end of bridge, to allow heavy vehicles to wait and pass. Ensure good visibility on approaches, so that decision to enter bridge is easy without mistakes leading to reversin off. If visibility is restricted on one side, then provide priority signs one way. Width between parapets may be determined by maximum load width; possibility of pedestrian and cycle use should also be considered. Upstand kerbs can be provided to restrict lane width. 4.0 m seems reasonable for this, with remainder of width as path either raised or not (if kerb is applied to deck) to allow for wide loads. ------------------------------------------- Andrew Irwin Auckland Transport -------------------------------------------
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  • 8.  RE:Single Lane Bridge Width

    Posted 12 May 2014 22:35
    Warren is correct, there are a lot of singl lane two way bridges in NZ. There are still even some on State Highway 1. Because of this and the long histroy of them their signage is detailed in the NZ Manual of Traffic Signs and Markings (MOTSAM) here:http://www.nzta.govt.nz/resources/motsam/part-1/

    You may find this useful, however the regulatory environment you are in requires coareful consideration. As a footnote, in my experience most lay people (and many road engineers) cannot interpret the meaning of the signs placed at the bridge, rather they know that the roaund one that looks a bit loike a speed limit sign means give way and the bue one means you have priority.

    As far as width goes, I agreee, 5.4 m is too wide. 4.2 (which is about a standard farm-gate width here), should be about right, but NZTA talks about 3.5m or so, which is often too narrow for farm vehicles.

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    George JasonSmith
    AECOM New Zealand Ltd
    Shortland Street
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  • 9.  RE: Single Lane Bridge Width

    Posted 15 January 2015 18:04
    Belated Greetings

    I have been reviewing this discussion, for new single lane bridges we are looking at in western areas.  The 4m referred to in AS 1742 is between edge lines and does not refer to bridge width (see Hugh's comment below); so no conflict with AS 5100.

    The main issue here is broad acre farm equipment, and in particular its track, where on older bridges with overall widths of around 4m the machinery runs on top of the kerbs and in some cases, on narrower bridges, flattens the railings.  Of the 8 criteria for not having a railing in AS 5100 we generally meet 7.

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    John Kauter
    Director
    EMC Works
    GOSFORD NSW
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  • 10.  RE:Single Lane Bridge Width

    Posted 01 May 2014 22:26
    Peter's suggestion is worth considering and even extending it to use technology to control heavy vehicles wanting access across older two lane bridges. Vandalism/theft may be an issue to keep the controls working. I understand that the maximum width in the Standard for a single lane bridge is to avoid two drivers in opposing vehicles thinking that they can both pass. A narrow bridge that looks narrow is an obvious deterrent. A nominal extra width may be required for pedestrians and cyclists. ------------------------------------------- Grant Sheldon Managing Director Sheldon Consulting Pty Ltd Northbridge NSW -------------------------------------------
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  • 11.  RE:Single Lane Bridge Width

    Posted 02 May 2014 23:53

    Hi Phil,

    I'm assuming there is at least one alternative route for the motorists to take.
    Well, in my opinion there isn't any reason not to consider a single lane bridge. As long as it meet the standards and warrants.

    On a past trip to northeast Italy, where I purposely decided to ignore the main roads and chose the local ones instead. 
    I came across several one lane bridges, over small rivers, creeks and canals. I suspect these bridges serve the local community more, than they do the general motorist or tourist.

    They mainly fall under 2 types:-

     

    • Controlled by signage; Speed, height, width and weight limits. The give way sign, of opposing directional small red and large black arrow.
      Although I did see one which an opposing directional red and black car.

    • Controlled by signals; Only saw one of these. It still had the same signage but also there was a radio controlled, 2 aspect signal. 
      On approach it was illuminated as Red, I was near to a stop when it changed to Green.

     

    The signalised bridge did provide me with greater reassurance overall, being a foreign motorist. So I'm in agreement with Peter that signalised one lane bridge is the way to go.



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    Angelo Zanatta
    Project Manager/Consultant
    Infomaster
    Clarence Park SAau
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  • 12.  RE:Single Lane Bridge Width

    Posted 04 May 2014 09:10
    Hi all, We've chosen to build one lane structures on low trafficked roads, due to the high costs of a two lane structure. In these circumstances we rely on sight lines, the obviously apparent single lane nature of the bridge, suitable pull over areas either side and appropriate sign posting. In affect, a risk management approach. This practise seems well accepted here, and in other areas we've travelled. Perhaps some of our NZ friends might comment as there appears to be quit e a few single lane structures in NZ. For the traffic volumes we have at these sites (typically less than 250vpd), we would not consider the extra costs of lights warranted, particularly as many of these are remote from natural surveillance. That said with the new solar units with sensors out there now (eg speed curve sensor warnings such as those we've installed on the Kings Highway), it could be a viable add-on for particular problem sites. With the number of timber single lane bridges we've got, let's not go crazy though and put it in an Australian standard as we simply can't afford it, nor would it be justified. ------------------------------------------- Warren Sharpe Director Infrastructure Serivces Eurobodalla Shire Council MORUYA NSW -------------------------------------------
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  • 13.  RE:Single Lane Bridge Width

    Posted 05 May 2014 01:51
    I agree with Warren Sharpe. As long as the bridges are properly sign posted as to who has right of way, and at each end a suitable area to pull off the roadway, then it should not be a problem. I was in the Daintree/Cape Tribulation area yesterday and there were a number of single lane bridges to cross, and I regret that neither the Cairns Regional Council nor Douglas Shire Council after de amalgamation have seen fit to sign post the bridges, causing consternation at each approach. ------------------------------------------- Ian Hudson Director TACTILE ACCESS CONSULTANTS PTY LTD AIRLIE BEACH QLD -------------------------------------------
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  • 14.  RE:Single Lane Bridge Width

    Posted 05 May 2014 01:52
    Thanks for your practical advice Warren. I think a risk management approach for rural councils with many single lane bridges and limited budget makes a lot of sense. ------------------------------------------- Phil Gee Director Sugden & Gee Pty Ltd LAUDERDALE TAS -------------------------------------------
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  • 15.  RE:Single Lane Bridge Width

    Posted 05 May 2014 03:12
    Hi All

    Signal control is not likely to be warranted for rural low-traffic roads, but one option may be Variable Message Signs, which can detect approaching vehicles and warn traffic approaching form the opposite direction, where visibility might be limited. These can operate with solar power.

    Norfolk County Council, UK carried out trials of VMS in a variety of situations, including speed discrimination to trigger additional warning of hazards. This type of sign can be of value where judgement of the speed of vehicles may be a decision factor. For example, long straight rural roads where who will arrive first at a hazard such as a narrow bridge may be hard to judge.

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    Andrew Irwin
    Auckland Transport
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  • 16.  RE:Single Lane Bridge Width

    Posted 05 May 2014 03:12
    Hi Warren,

     Given that you are actually constructing a new bridge, and retrofitting an existing bridge, you might find the extra cost of building-in the signal option to be surprisingly low.

    There are a number operating approaches including "resting on both red option".  This option means that a green signal (for a set number of seconds) will only be displayed when a car approaches, activates a detector and there are no opposing vehicles crossing.  With current low-power technology, you may only be talking about a few thousand dollars.  How much are you prepared to spend to mitigate the rick further?  That is a question only you can answer.

    Its been great participating in the discussion, hope it's been helpful for others.

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    Peter HARPER
    Town of Gawler
    GAWLER SAau
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