Design speed defines how fast cyclists can travel along the route section without endangering their safety.
High design speed means shorter travel times, and therefore increases the competitiveness of cycling.
Consistent design speed reduces the need of braking and accelerating (comfort).
Even the users that cycle slower enjoy the benefits of high design speed: the good view of the route ahead gives ample advance time to make decisions (safety).
How fast people cycle?
Historically, the design speed for main cycling paths used to be 30 km/h. There are two reasons to consider higher design speeds:
Design speed should be increased on slopes. First, cyclists can go faster on downhills than on flat sections; second even if they cycle at the same speed, they need longer distance to stop (gravity vs braking), therefore also longer sight distances etc.
The arrival of speed pedelecs created a demand for infrastructure accommodating faster cyclists, travelling up to 45 km/h.
What does it mean?
The main geometric parameters determined by design speed are:
Horizontal curve radius: the user should be able to travel the curves at design speed while keeping stable position in relation to the edge of the rideable surface. In most guidelines a typical 30 km/h design speed translates to minimum curve radius of 20 m, but e.g. Flemish “Vademecum Fietsvoorzieningen” recommends curve radii as high as 35 m. Catalonian guidelines vary minimum curve radius for 30 km/h between 24 m and 44 m, depending on the friction co-efficient of the surface.
Sight distance: the user should be able to see the surface of the cycle highway in advance to be able to analyse the situation and safely stop from design speed or make a manoeuvre in case of an obstacle. Usually comfortable sight distance is considered equivalent to 8-10 seconds of cycling at the design speed (e.g. 70-80 m at 30 km/h), bare minimum is 4-5 seconds (35-40 m).
Design speed affects also safety requirements on crossings: the drivers of vehicles that are expected to yield to bicycles on cycle highway should be able to see them from a distance that gives them enough time to react and decide. The requirements are usually expressed in terms of visibility splay - triangle that should be free from opaque obstacles. The triangle side along along the street (L1) is depending on the speed of the car at the crossing location; the triangle side along the cycle path (L2) - on design speed of the cycle highway. Exact values vary between national and regional guidelines (varying assumptions about reaction time, surface friction etc.)
Design speed vs average speed
Average speed needs to consider the loss of time from slowing down (because of a tight curve), stopping (e.g. to give way on a crossing without priority), waiting (e.g. for green light) and accelerating. Partial formula for calculating average speed is provided e.g. in “Qualitätsstandards für Radschnellverbindungen in Baden-Württemberg”, but the real loss of time on an interruption depends on e.g. how fast can the cyclist accelerate (which depends on the cyclists, but also type of bike). Therefore, many guidelines consider number of interruptions and waiting time on them as separate parameters.
min. horizontal curve radius
Design Manual for Bicycle Traffic (CROW)
30 km/h (40 km/h outside built-up areas)
20 m (for 30 km/h)
35-42 m (minimum), 70-84 m (comfortable)
Vademecum Fietsvoorzieningen (Flanders)
Kwaliteitscriteria voor fietssnelwegen (Vlaams-Brabant)
(8-10 seconds ride)
Qualitätsstandards für Radschnellverbindungen (Baden-Württemberg)
Mistakes to avoid
Applying minimum curve radius for a given design speed despite space availability to provide a more generous one.
Although several guidelines or manuals mention higher design speeds for cycle paths, most of them do not provide concrete geometric requirements above 30 km/h. An interesting exception is “The manual for the design of cycle paths in Catalonia”.
Table: comparison of cycle paths’ geometric parameters for design speeds 30 and 50 km/h according to “The manual for the design of cycle paths in Catalonia”.