Sharing the carriageway with fast moving motor vehicles is both a safety hazard and discomfort. But segregation can also have negative consequences, creating conflicts with perpendicular traffic or pedestrians. Most cycle highways mix stretches of segregated cycle paths and carriageways shared with motor vehicles, with degree of separation best adapted to local context.
The necessary degree of segregation between cyclists and cars depends mostly on vehicular traffic speed and volume. If motor vehicles are not frequent and drive slowly, cyclists can safely share the carriageway with them. As traffic speed and volume increases, the need for separation increases as well. Real and perceived safety might depend also on the complexity of traffic situation (crossings, parking, other manoeuvres), environment, driving culture or the volume of bicycle traffic (absolute or in relation to car traffic).
Minimum and recommended values
Guidelines and standards in different countries give different thresholds at what speed and volume of traffic separation of bicycles is recommended or necessary. As a rule of thumb, streets with 30 km/h speed can be considered safe for cyclists. But:
Cycling between large number of cars, even if they drive slowly, might not be comfortable, and in some cases also slow down the cyclists. Bicycles are not meant to be stuck in a traffic jam! According to the Design Manual for Bicycle Traffic, cycling on carriageway feels natural up to 500 cars/day, and might be comfortable up to 2500 cars/day if the number of bicycles exceeds the number of cars.
If cars are rare and there are not too many distractions around (e.g. outside built-up area), drivers have plenty of time to notice a cyclist and react, so limiting the speed to 30 km/h might not be high priority. In extreme case the reduced standard in Qualitätsstandards für Radschnellverbindungen accepts up to 800 cars/day and speed up to 70 km/h outside built-up areas.
Inside built-up areas
Outside built-up areas
Design Manual for Bicycle Traffic (CROW, NL)
bicycle streets only
Vademecum Fietsvoorzieningen (Flanders, B)
bicycle streets only
Fix the Mix (Flanders, B)
Qualitätsstandards für Radschnellverbindungen (Baden-Württemberg, D)
"very few" cars
Qualitätsstandards für Radschnellverbindungen reduziert (can be applied to 20% of route length)
London Cycling Design Standards (UK)
<200 vehicles/peak hour
Supercykelstier. Koncept 2.0. Planlægning, udformning og drift (Copenhagen, DK)
no perpendicular parking allowed, 2-1 roads can be a solution
Réseau cyclable à haut niveau de service. Objectifs et principes d'aménagement (Cerema, F)
Manual for the design of cyclepaths in Catalonia (E)
physical traffic calming measures
The decision to separate or not depends also on:
available width of the corridor
volume of pedestrian traffic
One needs to assess whether it is realistic to provide enough space for segregated cycling paths, separation from pedestrians and good visibility splays on junctions. A cycling path, where a cyclist comes into a crossing from just behind a fence might not be safer than riding in mixed traffic on a carriageway. On the other hand, without separated cycle infrastructure on a busy street some cyclists might decide to use the sidewalk. Below is the summary of main considerations for the available choices.
Segregated cycle paths
Width of the cycle path should be sufficient to allow overtaking between cyclists.