A to C via B

The first formal stakeholder meeting about the proposed cycle/foot bridge to link Abbey to Chesterton near the existing railway bridge over the Cam was held in early July.

The specific topic was the ‘bridge’ and ‘location’ rather than say ramps or approach routes. For any final decisions it is clear that approaches and ramps will have significant influences.

We were given some ideas of the expected trips such a bridge would carry in light of the Science Park Station and Wing developments. The range was around 5,000, that is twice that of the existing new Riverside bridge. A quick look at maps shows that those travelling to or from the Science Park, Cambridge Regional College, Marshalls, much of Abbey, Chesterton, Milton and even Cherry Hinton would benefit, and many trips would then be far quicker than by car. I believe the bridge can be justified even without the Wing and Science Park station developments, not to mention CB4.

I explored some of the issues regarding this location in a previous newsletter (see NL113), but some things are now clearer.

Any bridge must be separate from the railway one, with clearance sufficient for safe construction and maintenance. Network Rail will not, for obvious safety reasons, allow a connection to Fen Road that would significantly increase traffic over the level crossing. A bridge to the west of the railway is also unviable because it would land on Stourbridge Common and ramps would be extremely obstructive on the non-Common side.

So what was considered:

  • Two locations for the bridge, one as close to the railway as practicable and one a hundred metres or so downstream.
  • Three designs for the bridge, a ‘through truss’ like the railway bridge, a box girder, and a half through girder, although some favoured something more aesthetic.

I think few favoured a bridge further downstream even though it apparently links better with the existing path over Ditton Meadows. Such a bridge, even if of an ‘iconic design’, would be a huge blot on the landscape with the ramps difficult to fit in on the towpath side and a massive intrusion on the meadows side. So I have no doubt a new bridge will be close to the existing rail bridge with ramps down onto the triangle of Network Rail land on the towpath side.

Ideas for the design were more divided. An aesthetic design would cost far more, and would it not be wasted when obscured by the huge railway truss? I favour a through truss, for what I consider two good reasons:

  • It would ‘blend’ better with the existing structure.
  • Unlike the other two options, the depth below the deck will be a minimum. For every half metre of construction depth the ramps would need to be another ten metres longer. So for a box girder design ramps would be some 20 metres longer, and possibly difficult to fit in.

So what wasn’t considered, even if mentioned:

  • The width. With such large flows on bike and on foot, segregation will be necessary. For the foot section you need a minimum of 1.5m, ideally 1.8m. With these flows there needs to be space for an adult cycling with a young child alongside to be passed in safety by another. I reckon this needs a minimum of 3m, ideally 3.5m. When you add a 0.5m ‘edge effect’ adjacent to the cycle section we need an absolute minimum of 5m deck, but ideally 6m.
  • The ramps: I’m sure that on the Fen Road (towpath) side this is fixed, with only a clockwise or anti-clockwise spiral ramp to be decided. With a segregated bridge, and no doubt shortcut steps for those on foot, the ramps on the other side need to ensure those on foot don’t need to cross the cycle section.
  • The access: the obvious desire line for construction access, via the rear of the Beadle Industrial estate off Ditton Walk, will also be a clear desire line for many on foot or bike. Should not that temporary construction road become a permanent foot and cycle access? Adjacent to the railway embankment for much of the route, would it not blend in? It might even mean the existing shared-use path across the meadows could remain as it is.

What about the time scale?

Ideally this bridge should open at the same time as the Science Park station, but despite the delay to the station until May 2016 that does now seem unlikely. It will be important for all interested parties to reach a consensus such that any planning application receives good support from the majority of groups. Finally, just to throw something in to muddy the waters, I’d like to see a span ‘floated in’, part submerged, from adjacent to the old ‘Pike an’ Eel’. I’m sure if Stephenson could manage this some 150 years ago in the difficult waters of the Menai Straights our modern engineers could do the same on the placid Cam. The cost of getting a huge mobile crane and the span across Ditton Fields, with a necessarily enlarged access road, would then be removed. Perhaps the Cam Conservancy could similarly provide a barge to supply materials, concrete etc. to the work sites?

Jim Chisholm