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

Directness refers to both distance, and time. (A longer route may be more 'direct' in time if it bypasses junctions, or places where people need to wait). 

The CROW manual measures directness in terms of a 'detour factor' - namely, the distance between the points of departure and arrival, divided by the direct distance between these two points, as the crow flies. A detour factor of 1.2 is the recommended target, but in practice this is very hard to achieve, especially for shorter trips. 

However, detour factors should not exceed 1.4 or 1.5. 

Directness in terms of time refers to minimising the amount of delay at junctions. For all kinds of cycle routes, the amount of times people cycling have to stop, or do not have right of way, should be as close to zero as is possible, although in practice this will, of course, be hard to achieve.

In particular, cycling infrastructure should aim to offer an advantage in terms of distance and time over the driving network.

From 'Cycle Traffic and the Strategic Road Network' - 

Cycle networks shall serve all the main destinations and shall seek to offer an advantage in terms of distance and journey time.