One of the five main demands used to measure the quality of cycling infrastructure (the others are cohesiondirectnesscomfort and attractiveness).

Safe design for cycling should flow from the principles of Sustainable Safety. In particular, the principle of homogeneity should mean that cycling is separated as much as possible from motor traffic, and that where encounters do have to take place, the speed, mass and volume of motor traffic is limited. Roads and streets should have distinct functions, and should have predictable design. The consequences of mistakes and deliberate bad behaviour should not be serious; the environment should be designed to accommodate them. 

From 'Cycle Traffic and the Strategic Road Network' - 

Cycle networks shall not only improve cyclists’ and other road users’ safety, but also their feeling of how safe the environment is.