Why tunnels are better than bridges for cycling

A couple of weeks ago a campaigner from Cambridge in the UK asked me a question about bridge parapet heights in the Netherlands especially with regard to clearing railway lines. He'd realised that he'd not had any problems due to climbing bridges in this country and assumed that the Dutch had standards which were more suitable for cyclists than the UK.

However, the answer to this question turned out to be more involved than just heights of bridges. Actually, in the Netherlands there are not many high bridges. Cyclists in the Netherlands use tunnels and underpasses far more often than bridges. There are very good reasons for this which I'll explain below, but first a graphic showing the facilities which exist in both Cambridge and Assen to cross railway tracks and major roads which would otherwise form barriers to cycling:

Crossings marked with an X are cycle and pedestrian exclusive crossings. Note that all but three of the combined crossings for cyclists and motor vehicles in Assen have separate cycling infrastructure. Crossings of the river Cam and canals in Assen are not included though they make much the same point.  There are many canal bridges in Assen - mainly cycling specific flat opening bridges which do not require riding uphill and none have obstacles upon them. Assen's many crossings form important links in the fine grid of high quality cycling facilities required for a high cycling modal share.

The diagram above does not include
bridges over rivers and canals. No
bridges in Assen require dismounting
like this example in Cambridge.

As you can see, in both cities, the railway line cuts the eastern part of the city from the western part while major roads have a similar effect on the western parts of the cities.

The maps show crossings of motorways and ring-roads only, excluding rivers and canals as well as roads closer to the centre.

Comparison of crossings in Assen and Cambridge
It's immediately obvious that there are far more green crossings (tunnels) in Assen than there are red (bridges). The reverse is true in Cambridge. What's more,

The railway has a similar effect on both cities, cutting off people in the east from the centre. More people live east of the railway in Cambridge than is the case in Assen.  Note that in Assen all the most commonly used crossings are either tunnels or level crossings while in Cambridge the majority of crossings are bridges.

It's a similar story with major roads. Both cities have a motorway running north-south west of the city. Cambridge also has a dual carriageway (a road built to motorway standard) running west-east across the north of the city, while Assen has a partial ring-road which runs around the west of the city. These roads are crossed almost entirely by tunnel or level in Assen while they are crossed by bridges in Cambridge.

All crossings in Assen can be used
without slowing down. This is one
of the many cycle and pedestrian
crossings of a major road in Assen.
Four metre wide cycle-path, separate
pedestrian path, gentle inclines, well
lit and we can see right through for
 good social safety. Built in the 1970s
well maintained: last resurfaced 2012

Note also that in Assen the crossings mostly have an X which indicates that they are cycle-specific crossings. There are also crossings shared with cars, but these include separate infrastructure for cycling.

In Assen it is rare for a cyclist to use a bridge, common to use tunnels, and very often we cross on infrastructure which is cyclist specific so that cars are rarely seen. In Cambridge the crossings are mostly bridges, usually along the same routes as used by cars, and in several cases you have to cycle on the road to cross major roads or the railway line.

Dutch standards for Tunnels and Bridges

CROW still recommend maximum
of 5% incline and that's what this
tunnel has. Complaints from some
local cyclists have led to this Assen
underpass being redesigned at 3.5%.

The CROW Design Manual for Bicycle Traffic includes many details of how both bridges and tunnels should be designed to make cycling over and through them safe and convenient. I'm not going to repeat all of their recommendations here but will include some important points.

1. The incline to a bridge or tunnel should be less than 1 in 20 (5%)
2. Upward inclines: "Upward inclines require cyclists to make an extra effort and should be avoided where possible in the design of a bicycle friendly infrastructure."
3. Downward inclines: "On long declines, attention should focus on the speed of the descending cyclist". It is suggested that planners should expect  "35 to 40 km/h" and that there should be "plenty of free deceleration space at the bottom of inclines, with no intersections, sharp bends or other obstacles in the way".
4. Absolute minimum width of cycle-paths should be 3 m. That's permissible only if there's a separate 1 m minimum walking path on both sides of the cycle-path. Without a separate walking path (i.e. where no pedestrians are expected, this isn't shared use) the minimum width becomes 4.15 m, made up of 3.5 m cycle-path plus 0.325 m clearance between each side of the cycle-path and any railings or wall.

All the examples in Assen meet all these requirements except for one tunnel built in the 1960s which is a little too narrow.

Generally speaking, it is better that cyclists do not have to climb to cross roads or railway tracks. It is better to have cyclists continue on flat infrastructure and that powered vehicles should climb.

Only three bridges in Assen have a
significant inclines for cyclists. Most
are completely flat like this example.

Advice for bridges

1. Gradients should not be constant all the way up the incline. Cycling speed diminishes when climbing. For relatively short inclines (height less than 10 m), the highest section should be less steep than the lowest section to enable cyclists to maintain an almost steady speed uphill.
2. If a height over 5 m must be climbed, 'resting places' in the form of a horizontal section about 25 m in length should be provided before cyclists must to climb again.
3. Wind nuisance is greatly increased on an exposed bridge so this should be taken into account. Climbs against the prevailing wind should compensate by being less steep. Wind barriers can be installed on bridges to reduce the nuisance to cyclists.
4. It should be possible to cycle onto and over a bridge. Cyclists should never be required to dismount. Escalators or lifts to access the bridge are OK as a last resort measure.

Problems with bridges
The following are given as specific problems with bridges:

1. There are often longer inclines than with a tunnel (because of greater height difference in order to clear railway lines, for instance - precisely the parapet height question which prompted this blog post)
2. There is a possibility of fear of heights with a high bridge
3. Bridges must be designed to keep height difference to be overcome by cyclists as small as possible
4. Suggestion that with a cycle-bridge across the road: if necessary the road should be lowered to make the cycle-bridge less high.

CROW ideal tunnel impression. Short
open, well lit, separate pedestrian path
also of good width. Splayed out sides

Advice for tunnels

1. Steeper gradients can be used than with a bridge because cyclists going into a tunnel first ride downhill and pick up speed which can be used to climb back out of the tunnel.
2. Tunnels can be made less deep by moving roads and railways above them upwards.
3. Social safety issues should be addressed by making it possible to see out of a tunnel before you enter, and by avoiding long tunnels.
4. A "semi-buried" design can work well, with the road above rising by about two metres, effectively a small bridge. This makes the tunnel into an open structure and reduces the change in height required of cyclists.
5. Tunnels require good drainage (often pumped) and should be designed to be easy to clean.
6. Tunnel height should never be less than 2.5 m and width should be no less than 1.5 x the height in order that the tunnel feels comfortable to use.
7. Lights and light colours are preferable in a tunnel to make it appear as 'open' as possible. The time spent in a tunnel should be minimised and sides should be splayed outwards.

Some of the suggestions refer to social safety issues. In short, infrastructure should not lead to a feeling of unease, especially after dark.

All the tunnels were retrofitted to
Assen. The process continues. This
tunnel dates from 2008. Note that this
is an example of where the road rises
slightly as the cycle-path drops.

Why tunnels are preferred
CROW consider that tunnels are "often more favourable". They make many points including:

1. Tunnels have a smaller height difference than bridges. Only need clearance for the height of a cyclist, not for trucks or trains plus electric lines.
2. Tunnels take up less space than a bridge because inclines are shorter
3. Tunnels are easier to fit into an existing landscape.
4. Tunnels offer protection from wind and rain
5. Tunnels offer faster journeys than bridges due to less climbing
6. In rural areas tunnels can also be useful for wildlife

There are also other advantages which may seem to be quite small such as that tunnels naturally provide shelter when it rains.

Tunnel disadvantages
A possible disadvantage is low social safety. It is important that cyclists can see out of a tunnel before they enter it. There should be no turns within the tunnel, no-where for a potential mugger to hide. Obviously tunnels should also be well lit.

Drainage is very important in tunnels. The Netherlands has many tunnels which are below the water table and require pumps. Nevertheless, it is rare that tunnels become flooded.

The best tunnel in Assen is a bridge

Conceptually, this is an incline-less
tunnel for cyclists, not a bridge for
cars. It provides part of a direct and
uninterrupted route by bike from a new
suburb to the centre of Assen. This
bridge has no benefit at all for drivers,
only for cyclists. Re-opening the canal
for tourism was a side-benefit. This
replaced a large flat road junction.

If possible, it's best that cyclists don't have to change level at all. If motor vehicles can be sent into a tunnel or over a bridge then they no longer hinder cyclists.

In 2007, there was a traffic light junction at this location in Assen. For cyclists to use the road to travel directly into the city they had to stop at a traffic light. By 2008 this bridge had been built. It severs the pre-existing link by motor vehicle into the city, leaving the direct route as a bicycle road which excludes through motor traffic.

This bridge has no utility for drivers. It actually reduces their options as it is now impossible for a car travelling over the bridge to turn left of right as used to be possible.

Instead of building this bridge to carry four lanes of motor vehicles, a much smaller and less expensive bridge could have been built to take cyclists over the road, a small tunnel could have been excavated to take them under the road or a signal controlled crossing could have been installed on the level. However all these other options would have meant a reduction in speed and convenience for cyclists due to inclines for bridge or tunnel and delays at traffic lights for a level crossing. There could also have been social safety issues. The solution, to ensure the best possible service for cyclists was this bridge. Cyclists now have a smooth, level uninterrupted route which is well lit at night and has good sight lines in all directions.

Just as recommended by CROW, motor vehicles have to use inclines in this example rather than cyclists.

Short note about funneling

Illustration of how high cycle counts can indicate
a problem: A lack of bridges or tunnels to cross
railways, roads, rivers or canals can force people
onto the same crowded route. High cycle counts
are not a symbol of success when they result from
detours and traffic-jams for cyclists. A proper
comprehensive grid of infrastructure keeps counts
down. Not so good for photographers who want
to see lots of bikes but good for individual cyclists.

Unless enough pleasant routes are provided, excess numbers of cyclists are likely to be seen on the few remaining routes. It can be especially a problem where there are too few crossings of railway lines, major roads or rivers. Such funneling can make for great promotional headlines ("N bicycles per day pass this point") but actually it's not good news for cyclists at all because this actually means a detour onto overcrowded cycle-paths and conflict.

It is far better for cyclists that there should be more available routes so that more people can make direct journeys and there is less of a need to detour to find a comfortable route. Detours should be minimised by providing extra cycle crossings of large roads, railway lines, rivers and canals. This makes cycling more viable for more people and therefore more attractive. This principle should not only be applied for what are considered to be practical routes - CROW state that "recreational routes can also form reason enough to remove barriers".

Reducing funneling in Groningen
Groningen has many students, making up a relatively transient population who while they are more likely to cycle are also likely not to know the local area well. The city used specific marketing to encourage people to choose a selection of other routes which would serve them better. However, it's important to note that this was only possible because a very comprehensive grid of cycling infrastructure already existed.

It comes down to having a proper grid
I've often railed against hype about exceptional pieces of infrastructure. They're nice to see, but not really very important. The fact is that a few impressive bridges or tunnels are of relatively little use unless they form part of a comprehensive grid of good quality infrastructure. The grid is really the exceptional achievement of the Netherlands. The grid is the thing which should inspire and be copied elsewhere.

Tunnels are less photogenic than bridges, but they are preferable for the reasons explained above. However, whether tunnels or bridges are built it is most important that there are enough of them, that they are of high enough quality and that they link everything else together.

Find out more
Both tunnels and bridges feature on our study tours.

The Cambridge map does not include the Newmarket road roundabout underpasses as they do not cross railway or motorway. Nearly at the geographic centre of the map above, these underpasses are right not well loved. They are bad examples for a number of reasons include low social safety and sharp turns at the bottom of inclines. It also does not include the Northfield avenue underpass for similar reasons. This is flawed mainly due to dangerous railings within it. There are also many bridges in Assen which are not included on the map because they don't cross main roads or the railway, but all those which have a significant incline in Assen are on the map. The point of the maps is not to show all bridges and tunnels but to show red vs. green. i.e. emphasis on bridges in the UK vs. emphasis on tunnels in the Netherlands.