Bicycle Network: Good Design Guides
Boardwalks & Bridges
- Email me updates
- path, boardwalk, slip, wooden, timber, wet
Bridges and boardwalks help riders cross water, railways and roads that would otherwise require a long detour or a difficult or delayed passage. They often have a combination of curves, slopes, restricted space and reduced visibility that can increase the risk of crashes. A smooth, non-slip riding surface is critical.
What’s the problem?
|Paths built above ground on boardwalks and bridges are relatively expensive to build compared to those on the ground. There are also usually confined by space, and the options for approaches are often limited so there is often a combination of slopes, bends and restricted space and sight lines.
Rough or slippery surfaces are especially hazardous on a curve or slope where a rider has to change direction or brake.
Riders slipping or hitting broken elements often have serious crashes as they fall off the side of the boardwalk or into a fence barrier. Boardwalks built in hollows are more prone to becoming and staying wet or icy.
The boardwalk below at left on the Yarra Trail, had an ongoing history of bicycle crashes, including one lady who fell in the wet and broke her arm. It has been replaced (at right) with a smoother, safer (though still slippery when wet) surface and better fencing.
What are the risks?
The main risks to path users on boardwalks and bridges are:
- hitting or coming into conflict with other path users due to the narrow path
- not seeing other path users or obstacles due to low visibility or short sight distances and crashing into them
- hitting obstacles, including fencing, on the side of the path and crashing
- slipping on, or being unable to stop, due to a rough or slippery path surface, and crashing
Crashes on boardwalks and bridges are often serious as riders can fall some height off the side of the boardwalk or into the fence barrier along the edge. In Melbourne there has been at least one death of a bike rider who fell from a boardwalk (in 2010).
If the boardwalk or bridge is non-durable the responsible authority can face an expensive on-going maintenance regime to ensure a safe riding surface – for instance to replace wooden slats that have lifted or re-apply a non-slip surface.
What is the solution?
Curves, slopes and narrow paths are often unavoidable on bridges and boardwalks and their approaches given confined space and limited path alignment options. Path design should reduce the risk as far as possible by minimising the combination and magnitude of these potential hazards. For instance if a boardwalk needs to be narrow and curved then slopes should be avoided, sight lines kept clear, fences kept high and a smooth, non-slip riding surface provided. If a path is sloped and curved then it should be kept wide, non-slip and well defined with clear sight lines.
In all cases a smooth, non-slip surface is critical. For bike riders a non-slip surface equates to a mean Wet Pendulum Test value (BPN) of 60 or more. Under the Australian standard for slip resistance for pedestrian surfaces (AS 4586:2004, AS 4663:2004 and Handbook 197:1999) this coefficient of friction level is rated as Class V slip resistance - the recommended slip resistance class for ramps in outdoor areas (that may get wet) for pedestrians.
The Wet Pendulum Test gives a measure of coefficient of friction for the surface using a standard testing procedure. A report by VicRoads on a performance criteria for bicycle paths recommended the above friction level and class and they are borne out by rider experiences. See Building Better Paths That Attract Riders. The recommended slip resistance for flat outdoor pedestrian areas (Class W materials with a BPN mean value of 45-54) may not provide enough grip for bicycle tyres under all circumstances - for instance the surface may not be grippy enough to prevent a rider falling while braking to avoid a dog or other hazard on the path. Materials tested as Class W slip resistance have been reported as slippery to bike riders, especially when wet.
Concrete is a proven performer as the sand cement provides an integrated non-slip surface. Concrete usually also requires a properly engineered substructure to support the path that may be less likely to shift over time. On the minus side, concrete boardwalks tend to be more expensive, heavy and difficult to install in remote or areas with restricted access. Other surfaces need to be used with caution as they are prone to become slippery and the surfaces tend to less durable. Life time costs may be more expensive for non-concrete surfaces given the extra maintenance and replacement costs to maintain a safe riding surface.
Wooden surfaces tend to be slippery, especially in the wet, and usually require an additional non-slip surface treatment to prevent cycle tyres sliding under wet conditions. This can include an asphalt overlay or sandy compounds over the top of the wood. Chicken wire fencing tacked to the surface of wood boardwalks is not effective as a non-slip surface - it may become loose over time and is not significantly less slippery than wood.
Wooden slated surfaces are rough for riders and individual boards (even those secured with screw fittings) tend to lift over time with the continual loading and unloading from bicycle traffic. It seems that many wooden boardwalks have been engineered for mostly walking traffic that has different loading patterns and walkers are more able to deal with small steps than people on bikes.
Wooden boards, if used, should be installed perpendicular to the direction of travel as bicycle tyres will get caught or deflected by the gaps between boards that are parallel to the travel direction.
Newer composite materials made may be appropriate as an alternative to timber as they can be made with a non-slip surface and can be more easily installed than timber in some cases and lighter and more easily transported than concrete.
An online survey conducted in September 2010 amongst riders by Bicycle Network Victoria indicated that most reported crashes or incidents on boardwalks are caused by slipping on the path surface in the wet. Wooden surfaces rated poorly amongst users, especially when wet and the addition of chicken wire did not improve their performance markedly. Riders preferred concrete surfaces for comfort and safety.
What do the guidelines say?
The Austroads Guide to Road Design – Part 6A, Pedestrian and Cyclist Paths (2009) is clear on the importance of bridges and boardwalks:
"The design of structures is very important to cyclists. Existing road bridges are often narrower than the road on the approaches thus creating a squeeze point for cyclists. Because of the high relative costs of new bridges there is an understandable tendency for designers to be as economical as possible in the widths provided for various users. It is important, however, that road managers look for ways to better cater for cyclists at all existing structures and that designers and planners ensure that cyclists are adequately provided for in the design of all new structures."
The Guide is also clear on the importance of providing adequate room, clear sight lines and a smooth, non-slip surface:
"The primary requirement of cyclists using bridges and underpasses are that:
• adequate path width and horizontal clearance to objects (walls, safety barriers, kerbs, fences, poles, street furniture etc.) is provided
• adequate vertical clearance is provided, particularly in underpasses
• good sight lines are provided into and through structures
• the surface is smooth and not slippery under any conditions; a particular issue can arise with expansion joints that can provide a rough ride and be slippery when wet and designers should seek better methods and materials to address this issue."
Unfortunately the Austroads Guides to Road Design are silent on how to specify slip resistance for bicycle riding surfaces. The emphasis is on providing a smooth enough surface but there is no mention of what constitutes a slip resistance surface for cycling. See our recommendations above which relies on Australian Standards for pedestrian surfaces (AS 4586:2004, AS 4663:2004 and Handbook 197:1999 An introductory guide to slip resistance of pedestrian surface materials); feedback from cyclists; testing on bikes; and other reports to provide a recommendation regarding slip resistant surfaces for bike riding.
Surfaces for cycling in general is covered in section 4.2.3 of the Austroads Guide to Road Design – Part 6A (2009) (p24-25). This includes tolerance for surfaces used for cycling. Parallel to the direction of travel, a groove should not be wider than 12mm and any step no higher than 10mm. Perpendicular to the direction of travel, any step should be no higher than 20mm. Grooves perpendicular to the direction of travel should be considered as two separate grooves if more than 100mm apart. See our page on surfaces for more information.
From Part 6A Appendix B.4.5 - Timber Surfaces (p 95)
Gaps between longitudinal planks in timber bridge decks (see Figure B 9) can trap bicycle wheels and cause serious injuries to cyclists.
Consideration should therefore be given to remedial treatment of existing timber bridges such as through an asphalt overlay of the outer 1.0 metre sections of deck to provide a smooth, safe ride for cyclists. At the very least warning signs should be provided on the approaches to bridges which have longitudinal gaps in the deck.
On new timber bridges the planks should be placed perpendicular to the direction of travel of cyclists. In constructing and maintaining bridges it is important to ensure that the deck joints at abutments and piers provide a smooth and hence safe passage for cyclists.
Drainage ought not to be a problem when one considers the number of gaps in the decks of timber bridges. However, individual planks have the potential to warp and collect small, localized pools of water. Timber surfaces can be slippery in wet or shady conditions. Where these circumstances are common, the application of a non-slip finish as above is also desirable, regardless of the alignment of planks.
What does Bicycle Network Victoria say about the guidelines?
The guidelines are clear on the need for a smooth, non slip and durable surface that retains its safety in all weather conditions. There needs to be clearer guidance on specifying a non-slip surface however.
The primary requirements of adequate width and clearance and good sight lines are indisputable.
Any examples, good and bad?
|Yarra Trail – rough wooden boardwalk that becomes slippery when wet. Cyclists will tend to ride on concrete path at left, bringing them into potential conflict with people walking.|
|Main Yarra Trail - floating concrete path. Surface is smooth and gaps between concrete sections are smooth enough to not be a hazard or cause discomfort. Fence barrier reduces usable width and could have been mounted on outside of concrete section not on the surface of the boardwalk.|
|Floating concrete path transition - Main Yarra west of Cremorne Rail Bridge. Ramp up is timber with non-slip surface treatment - as below. Floating boardwalk is concrete with plastic infill wedges at turns. Point of hazard is at bottom of the ramp on the steel plate where bike tyres are prone to slip due to increased gradient and slippery surface. There is a history of serious crashes here and at similar sites.|
|Main Yarra Trail at Mary St – ramp down at left is wooden boards with non-slip surface treatment. There have been crashes at junction at bottom of slope as bicycle wheels slip on ramp at transition. Ramp at right is steep and slippery in the wet. Note signage indicating conditions. These areas can become icy.|
Eastlink trail at Mullum Mullum Creek - Here the path, on a boardwalk as it crosses a gully, goes from concrete path (foreground) to grooved recycled plastic planking (middle) onto wooden planking. In the wet the concrete provides the best grip followed by the grooved recycled plastic planking then the wooden planking. Recycled plastic planking (with grooves as the smooth planks are slippery), precast concrete and other recycled products may provide a viable non-slip surface as an alternative to timber planking which, though easy to construct, is slippery in the wet without some sort of surface treatment.
19 March 2013 - we recently had reports from riders slipping on the grooved recycled plastic planking which we would not classify as non-slip for bike riders (BNP >60 or Class V rating). A non-slip surface post construction surface treatment is required.
Figure 11.2 from Austroads Guide Part 6A (2009).