Sunday, 15 April 2012

Adelaide O-Bahn

On Friday I travelled on Adelaide's guided busway, the O-Bahn. Its success was one of the reasons we in Cambridgeshire were given a busway rather than a reopened railway. You can read about the O-Bahn here and here. The Cambridgeshire and Adelaide busways are very different in terms of length, volume of traffic, design and speed. The Adelaide route is shorter than Cambridgeshire, but there are no flat road crossings requiring buses to slow down frequently. Roads and footpaths are carried over or under the O-Bahn on bridges, but there is a break at the intermediate stops. A yellow line is provided to help drivers to join the guideway efficiently.
Top speed is 100kph compared with 90kph in Cambridgeshire.
The track is constructed differently. Unlike Cambridgeshire it is not laid level with the ground, but is raised up by a metre or so on conrete piles which are sunk into the ground by as much as 4 metres. The piles in turn support transverse sleepers upon which the track is laid. I felt the ride was smoother than on the Cambridgeshire busway, which may be to do with the way the concrete tracks are supported.
Bus service frequency is higher in Adelaide than Cambridgeshire, and currently it is carrying around 8 million passengers a year. Adelaide runs only single-decker buses, including some bendy-buses. There are no double-deckers. As in Cambridgeshire, O-Bahn services are affected by road conditions off the busway, but Adelaide has wide streets, and the city centre has a grid pattern, so some of the problems that beset Cambridge seem not to be replicated in Adelaide. There are only 3 stops on the O-Bahn - Kemzig, Paradise Interchange and Tea Tree Interchange (they are more like mini bus stations than 'bus stops')- and the guideway breaks off at these points. This is a view from Tea Tree Interchange looking toward the start of the busway.
It is possible for express buses to overtake slower services at these places. Timetable boards are provided at stops but there are no real-time infomation displays which have proved so useful to Cambridgeshire busway travellers.

8 comments:

  1. I have noticed some "dipping" at the end-beams on the Cambridge busway. Are these recent, or has the ride always been a bit "up and down"? - noticeable only on the double deckers, I might add. My conjecture is that in places the track is now supported by the posts, in place by the ballast, inevitably at a different height.

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  2. I think that up and down motion has been there since the beginning, and it's definitely more noticeable on the double deckers. I have never been able to make up my mind whether the jolting is cause when the bus wheels pass over the joints between the beams, or whether it's to do with the beams sagging. Have you noticed that there are cracks in the surface of the beams at each mid point?

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  3. "Have you noticed that there are cracks in the surface of the beams at each mid point?" - yes

    However, more significant are the height differences between adjacent beams - 2 to 5mm as I measured south of Histon station.

    Equally, if you look out the front (from upstairs) the jolts are clearly synchronised with each new beam segment.

    Interesting that you say the jolting was always present. The COunty indicated a perfectl smooth ride. Perhaps they rode on a single decker.

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    1. That could be the explanation. Single deckers jolt less than doubles - but I'll pay attention on my way homne this evening.

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  4. Also, you suggested that top speed on Cambridge busway was 90kph -- "56mph" - but buses often travel much slower than this, particularly on curved sections, and when it is windy and bus motion seems a bit "lively".

    How straight is the O-bahn busway?

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    1. The O-Bahn is mostly on curves, and hardly any straights. It doesn't have the succession of level road crossings that we have here in Cambridgeshire, and so the buses can keep belting along. I once checked how much of the journey our guided buses travel at their top seped - it wasn't much - possibly about 5 minutes total. Take Oakington to Longstanton, for example. They can't really get going until they are past the uncompleted junction for Northstowe. Top speed isn't reached until the end on the straight sectuion and the start of the long sweeping bend. From there it's only about 90 seconds until they have to start slowing down for the Longstanton stop.

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  5. For the record on this blog, the St Ives - Swavesey section of the Cambridge Busway is also laid on piles. The difference is that in Adelaide the posts are more frequent - reports vary between 3m and 5m - and that it is possible to adjust the height of the beams, in case the piles slip.

    In Cambridge, the piles are every 7.5m (ends and middle of each beam). For the rest of the busway, where the ground conditions were considered more stable, the piles are replaced by large concrete 'pads'. The beams do not rest on the ballast at all, even though it looks as if they do.

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  6. interesting - "O-bahn is mostly on curves" - (tacitly) you suggest that the Adelaide busway is much better at handling the curves than the the Cambs busway - you certainly can't "belt round the curves" there - with double-deckers I think you have to be particularly gentle...


    Causes of "jolting" - I think it is a combination of sagging - there are 2 sags per beam, with a middle support. When a DDecker is going slowly, one can feel these sags, but as speed builds up, the frquency of this oscillation exceeds the buses natural frequency, which then begins to pick up the ends of the beams (half frquency) which are pointing slightly upwards. With the buses back wheels following a different sag pattern, the bus rotates back and forth. The suspension of the Sdecker snooths this all out. When I went today, the ride south of Histon was positively "exciting" (upstairs) and in general is getting very pronounced.

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