Saturday, 10 August 2019

Bridging The Gap

Whether it is a natural feature or a man-made barrier, we sometimes need to build bridges to get our cycle routes across them.

Now, I am not talking about crossing a road at surface level here, I am interested in rivers, large roads and rail where we need to be using a bridge of some kind and for those moving under their own steam, the key issue is one of energy and minimising effort expended in getting across the obstacle.

The best solution of course is to keep the crossing level so that no energy is lost. To do this, we either have to raise or lower the obstruction. If not, we'll have to use ramps to get over or under the obstruction or possibly meet halfway. 

The photo below was taken in Malmö and shows the road and cycle track having to dip under a railway. The thing about railways is they are most efficient when built flat and unless you can rebuild a long section (which does happen sometimes), then you have to work with what you have.

Going under something is quite good for cycling because you don't need too much headroom - about 2.5m is enough. The example above shows the contrast between cycling and general traffic with the latter needing about twice the clearance. To ensure we don't lose too much energy, slopes are very gentle. We can even do this in the UK as shown in the photo below taken in Stevenage.

Even if we have to leave the obstacle alone, dipping under it with long ramps isn't terrible, even when they are perhaps a little steep such as this example from Harderwijk;

Of course, there are much nicer Dutch examples such as this one on the outskirts of Amsterdam where the motorway is on an embankment;

There are obviously economic considerations in terms of raising or lowering infrastructure, but if we aim to keep people cycling at grade, then we'll provide the best outcome. If we are building a new motorway (or rebuilding a junction) we may be able to lift the levels a bit which means we don't have to drop the cycle crossing as much.

The sketch above shows three arrangements. The top is where the cycle route is at grade and the road raised. The second is half-way and the third is keeping the road at grade and dipping the cycle route. As you can see, the lower we drop the cycle route, the longer the ramps need to be.

So far, I've talked about the obstacle being on a bridge, but if we shift it the other way round, ramps become more critical. Of course, the obstacle being crossed may be at a lower level than the cycle route and we don't need to worry about it. In the Netherlands, there are many canals and so many bridges. They range from those with very long ramps to get over them because they are busy shipping channels;

To those where there are only occasional movements by boats and so opening bridges are used; 

That's all well and good, but railways, roads and shipping routes are not really suitable to run in this way and so we need to get the cycle route up to a level where the obstacle is properly cleared. Again, we can play with levels so that we minimise the need for ramps. Having the obstacle in a cutting is the obvious starting point such as here where the A1 motorway near Amsterdam dips down under this cycle track and road;

Sooner or later we are going to have to raise the cycle route over the obstacle and this is going to mean long ramps. In the UK, we often end up with zig-zag ramps which are awkward to use and really interrupt the flow of cycling as as this one in Salford;

However, we can actually manage to do a half decent job when we put our minds to it. In the week, I put out a tweet asking for examples of curly bridges where spiral or curved ramps are used to try and make the user experience nicer. Of course, the ramp lengths are the same as a zig-zag bridge, but curves are easier to steer around than corners.

There were lots of wonderful suggestions, but I think I am going to pick one in Hatfield because it was actually slap bang opposite where I lived for a year and I cannot honestly remember it!

Now, there are plenty of bridges with a spiral on each side, but this bridge actually carries a proper cycleway from back in the days when we knew how to build them. Thanks to a couple of photographers, we can see a stepped cycle track;

If you have a look at the section of road, the stepped track continues, but quickly becomes more conventional UK fare. There is a tantalising hint of history and an implied zebra crossing a bit further up;

So, lots of ways to do it an despite the fashion for surface level crossings, grade separation must still have some relevance even if in some cases it's a bit of getting cyclists out of the way of drivers - at least there's no waiting around for a short green.

1 comment:

  1. The cycleway leads down to what was Southfield school near the last picture which only closed in 2013 (presumably kids running for their lives accross or put a signalised crossing on the A1001 South Way didn't appeal) It doesn't look too old so may have built after you'd left Hatfield.