Surface quality

Why is it important?

• The quality of surface affects the energy expenditure of a cyclist. For example, compared to a good quality asphalt, paving blocks require 30-40% more energy per kilometre and gravel surfaces 50-100% more energy (see UPI-Bericht 41). On network level, bad surface material or maintenance means that the area reachable with a set energy budget is reduced 2 to 4 times.
• Uneven surfaces can create a risk of accident, e.g. a cyclist losing balance and falling after riding into a pothole.
• Poor friction makes it risky to perform even simple manoeuvres, like braking or turning.
• As most bicycles have only two wheels and narrow tires, they are more vulnerable to bad surface than cars.
• A cycle highway should be usable by fast bicycles (racing bike, pedelecs, velocars), that require high quality of surface.

How can it be measured?

Several different methods can be used to assess the evenness of surface:

• Visual inspection of cracks, holes, bumps etc. against a list of standard defects;
• Ruler and wedge applied to uneven surface to measure the scale of defect;
• Road profiling by laser sensors;
• Bicycle mounted accelerometers directly measuring forces transferred to the cyclists;

In the first category, EuroVelo European Certification Standard provides a common framework for assessing the quality of different surfaces. But because of the wide range of surface materials and quality it covers, it has limited precision in the range that is really suitable for cycle highways.

Apart from evenness, another important characteristic of surface is coefficient of friction. It is clearly defined, but the measurement might depend on the tire used or whether the surface is dry or wet.

How is it related to other criteria?

• Parameters related to design speed, such as curve radius or stopping sight distance need to be increased on surfaces with reduced friction (non-asphalt or poorly maintained).
• Not taking these into account increases safety hazards related to crossings, slopes or obstacles.
• Gravel sections can reduce the attractiveness and comfort of the route by creating dust in dry weather conditions or mud in wet.
• Consistent colouring of the surface and its continuity on crossings can improve readability of the route.
• Dark surface colour merging with the surroundings increases the need for horizontal markings and/or lighting.

Uncharted areas

There seem to be no clearly established standard on how the surface quality measurements should be performed and results quantified. Laser sensors can be mounted on cargo bikes, bicycle trailers, mopeds or microcars, and profile surface in different parts of a cycle path. Accelerometers can be mounted in different locations (e.g. handlebar, stem or seat post) on different bicycles with different tires or tire pressures. Results from different countries or even different municipalities are currently not comparable.

For motorised vehicles, methods of calibrating and processing the data have been developed, to create International Roughness Index (IRI). However, IRI is calculated using a quarter car-model, reflecting mass, tire size and suspension characteristics of a motorised vehicle, therefore it does not necessarily describe well the impact of the surface on cycling experience. Bicycle models are country- or region-specific, no similar international standard exists up to date for bicycles.