Table of Contents

5.1.2 Definitions

Table 5.1 provides definitions of key terms as they are used in the development and application of gap acceptance analysis.

Table 5.1: Definitions of gap acceptance terms

Term Definition
Major and minor traffic streams In a gap acceptance situation, the traffic stream that has priority is called the major traffic stream and the traffic stream that must yield right of way is called the minor traffic stream.
Unit (in a traffic stream) An element of the traffic stream (vehicle, pedestrian, bicycle, etc.) moving as a single entity.
Major and minor roads At a road intersection operating by gap acceptance, the road carrying the major traffic stream is called the major road and that carrying the minor traffic stream, the minor road.
Gap acceptance point The point at which a minor traffic stream unit can commence its desired manoeuvre when a suitable gap presents itself (e.g. the arrival of a minor road vehicle at the stop line, or a pedestrian at the kerb).
Lag Time interval between the arrival of a minor traffic stream unit at the gap acceptance point and the arrival of the next major traffic stream unit.
Gap Time interval between the arrival of two consecutive major traffic stream vehicles (i.e. a major traffic stream headway), commencing after the arrival of a minor traffic stream member at the gap acceptance point.
Critical gap (critical lag) The minimum gap (lag) acceptable to a minor traffic stream unit to perform a given manoeuvre in a given gap acceptance situation. (In the discussion in this section, it is assumed that the critical lag and the critical gap are of equal size for any given situation and are the same for all road users at all times.)
Follow-up headway The minimum additional duration of a major traffic stream gap (or lag) required to allow one additional minor traffic stream unit to follow the unit preceding it into the same manoeuvre, utilising the same gap (or lag).
Anti-block That part of a sufficiently large gap or lag during which there remains a time at least as large as the critical gap before the end of the gap or lag, so that a minor traffic stream member could commence its desired manoeuvre.
Block A time interval during which zero, one or more major stream traffic units may pass, while there always remains a time less than the critical gap before the arrival of the next major traffic stream unit, so that a minor traffic stream member could not commence its desired manoeuvre.