Route interdependency produces network synergy

A well-connected public transport network yields greater accessibility than the sum of its individual routes. This synergy is due to the routes’ dependency on one another. A connective network builds on transfer opportunities between individual routes, which eliminate redundancy and increase efficiency.

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Frequent rail and bus network forms the organizing framework for metropolitan growth, in which inefficiencies are eliminated through making frequency as the primary hierarchy

Unlike the private transport network which operates without any time restriction, a public transport network faces ‘temporal fixity’. A true connective network effect is generated when transfer wait times are kept to minimum. Thus, a connective network must consist of a ‘network core’ of frequent routes that are connected to one another.

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‘Horizontal’ synergy occurs through connective synergy between routes of modes within a similar hierarchy (i.e. frequent streetcar/bus, or at-grade light rail/bus rapid transit), and ‘vertical’ synergy takes the synergy approach further (i.e. bike lane > frequent bus > metro) throughout the entire transit hierarchy spectrum (bottom image derived from Translink’s Transit Primer)

Balancing Coverage and Ridership Goals

Public transport network is bounded by both spatial and temporal fixity, and running frequent and schedule-less public transport to cover every single car travel pattern is not realistic. Constrained by limited resources, transit agencies are typically torn between expanding coverage (to ensure social equity) and increasing frequency (to increase mode share). Private-to-public mode shift potentials differ across different parts of any urban region. The trade-off between coverage and frequency needs a clear understanding on how tolerable would the consequences of these goals be. A balanced goal should reflect a frequent network core and a secondary social route network to plug equity-based coverage gaps.

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A purpose driven network (as coined by Jarret Walker) considers both efficiency-based ridership and social equity-based coverage goals