Sunday, 15 December 2013

Underground, Overground: Should We Be Building Underpasses & Bridges Again?

a few years back, i was working on a project to refurbish an underpass which was to be used for walking and cycling to avoid crossing a busy dual carriageway.

OK, we probably need to wipe the walls with a damp rag.
The underpass had been built years before in anticipation of a development which didn't happen and so it was blocked off and left to rot.

When we opened it up, apart from being waist deep in stinking water, it was in pretty good condition. It needed to be cleaned out, required rewiring and new pumps put in, but was structurally fine. New underpasses (subways) have not been in fashion for a few years because of maintenance problems, antisocial behaviour and personal security. These are real issues, but often a result of the design. 

CS3 gently dips as the A13 rises to meet the A406 to this pretty
good underpass which has a clear view through and no easy
hiding places for those up to no good.
On the positive side, they can be used to cross a road without having to wait for traffic signals and they don't hold up traffic (both of which may well be valid issues). They can also be more direct for pedestrians and people on bikes negating the need to divert to a surface level crossing which may well be staggered.

In high speed situations (50mph and higher) stand alone signalised crossings are not safe (in my opinion) because despite vehicle detection, the risk of a driver jumping the signals is high. Additionally, there is a risk that crossing users see the green man/ bike and assume it is safe to cross (it is an invitation to cross and people still need to remain aware of traffic conditions, regardless of the green signal) and so underpasses (and bridges) may well be the only safe option.

Pedestrian and cycle bridge over Rotherhithe New Road which
provides pretty much a level route high above a very busy road. 
Bridges solve the same issues and are perhaps less likely to be secluded compared with underpasses. They work best when coincidental to the route, in other words, they are part of the route and users don't have to deviate to access them and there are not any steep slopes. Like well-designed underpass, the roads being crossed will be the part of the infrastructure changing level as vehicles have engines and are not affected by level change as person power is!

The A406 in North London. Just one of a switchback approach ramp
to a footbridge which creates a route perhaps five times as wide as
the road being crossed.
Image from Google Streetview.
Of course, there are huge roads around the country which have severed continuity for pedestrians and people on bikes. Where bridges or underpasses are provided, they will have long, sometimes zig-zagging ramps and will not be conveniently following desire lines.

The general term to describe underpasses and bridges is "grade separation" - a regime where we are really separating people and traffic by space. Grade separation of this nature does come with problems;

  • If the crossing is arranged with long ramps this is an access issue for all users, but especially people with reduced mobility. Some users may just take their chance at road level, which is an issue where drivers simply do not expect to see people crossing.
  • Personal security can be an issue with underpasses and sometimes bridges where natural surveillance is not possible, again leading some to ignore the provision.
  • A great deal of space might be needed for the footprint of the structure, especially is ramps are being provided. Underpasses are not particularly intrusive in the streetscene, but bridges can be.
  • Bridges and underpasses are costly to build (underpasses more so as drainage often needs pumps) and maintain.
If a new road layout is being planned, then it is of course much easier to design grade separated crossings from the start. But what if we are faced with a multi-lane, high speed dual carriageway that needs crossing and we have to use ramps? What things can we do make things a little easier?
  • Make the underpass or bridge wide enough to be used by both pedestrians and people on bikes - people will cycle through them anyway, so assume they will and design this in from the start,
  • For an underpass, rather than provide one ramp and one set of steps each side of the road which is often the way things are done, provide two ramps which will cater for the desire lines of all users,
  • For people cycling, remember that they have handlebars and so design in protection to reduce the risk of clipping the underpass wall or bridge sides as this can easily throw people off.

  • Regardless of the quality of the route either side of the bridge or underpass, it is worth designing to provide separation - a kerbed footway at a level higher than a cycle track would be good practice as especially on ramps, cycling speeds can be a big issue for pedestrians.
  • Don't forget that people on bikes need more headroom than pedestrians. 2.3 metres is a minimum, but how about 2.5 metres or perhaps more to try and give a feeling of openness. Any lighting needs to take headroom into account!

Like any piece of infrastructure, some good thought at design stage can really make a difference and getting a bridge or underpass wrong will spend a great deal of money for something which doesn't get used and even worse, people will just cross the very road we trying to help them avoid.

There are many locations where people struggle to cross and indeed, designers struggle to accomodate people safely at road level. For me, the answer is grade separation and this something to  be tackled head on, even though it will need a substantial budget.


  1. I find often grade separated crossings are unfair to peds and cyclists, it is rare to see a road lowered to accommodate a bridge without an extreme ramp och a road raised slightly so the underpass does not require cyclists and pedestrians to go downhill followed by an up hill (and having to often deal with water puddles).

    Every time I have to take a huge detour because of an offramp where the designers didn't take bikes into account, I'm frustrated. I wonder if we will see a change in this thinking?

  2. And that it the problem. The CS3 example is great as the cycle route dips a little bit, but gently and over a couple of hundred metres. The A13 and A406 roundabout is built on an embankment and so it is the traffic which climbs. Steep and zig-zagging ramps are a nightmare to users, especially people using mobility scooters and bikes.

  3. "and are affected by level change as person power is!" should be "and are *less* affected by level change as person power is!

  4. Motorway interchanges are often multi-level because it keeps the traffic moving, building bridges and underpasses is really just the same - it keeps the traffic, bike and pedestrian traffic included, moving. Like you say, underpasses need to feel safe but they are a vast improvement where a busy road might as well be a 200ft ravine if you want to get across it.
    I hope they do come back into fashion, I can think of plenty of places where I live where there is no safe and easy way of crossing, which then doesn't encourage me to go anywhere near them, especially if I need to get from A to B with kids.

  5. As a highway engineer I can agree with everything you've written. It's all common sense. However, you will get nothing because people elect certain politicians who put motorised traffic first and they get the cheapest solution that suits vehicles. Until folk realise that they have to vote for the only party that puts the vulnerable at the top of the agenda, I'm sorry to say you can talk until you're sick of the sound of your own voice.

  6. Luckily, I like the sound of my own voice ;-)

    As I state in my "Why This Blog" page, this is my therapy in coming to terms with working in local government.

    I find things very frustrating as do colleagues all over the UK, but I am too far along my career to change my views about how the road network needs to be rearranged. We have tried it one way over decades which hasn't worked and so let's try something else.

    Keep fighting the good fight!

  7. Keep up the good work!

  8. There's nothing wrong with a good underpass. As you said, the problems with underpasses are often caused or compounded by bad design.
    The Dutch have gotten pretty good at them in recent years, by doing things like:
    - graffiti-resistant paint
    - good sight lines on the approach inclines
    - a daylight gap between carriageways where possible
    - most importantly: sloped sides, which makes graffiti less interesting, gives kids nothing to lean against and makes the tunnel feel a lot wider than it really is (do hit the "discotunnel" link on that article, btw)

    1. Fair comments. Sloped sides are more expensive (more tricky structurally) and need more land, but again remove hiding places - the disco tunnel is cool!