Proposition One: Telecoms is a possible substitute for transport. People switch from using aeroplanes to using teleconferencing according to a bundle of factors including price, quality, convenience.
Proposition Two: Telecoms often stimulates air travel. We encounter people and initiate activity through telecoms, which then leads to increased demand for face-to-face meetings. Telecoms and physical transport are therefore not substitutes but complementary.
The Telepocalypse weblog contains a useful discussion about these two propositions. Note how the discussion ranges across several domains.
Economic FactorsElasticity. Absolute changes in the price of a commodity, changes in the price of one commodity relative to another commodity, changes in the price of a commodity relative to disposable income.
Note that air travel is getting cheaper. Is this partly a response to the increasing availability/popularity of teleconferencing?
It is possible that the rate at which these prices change over time might also be important - because short-term price movements may affect people's perceptions that something is cheap or expensive, as well as expectations of future prices.
Social FactorsDiscussion of the social context for transport or telecoms - being there, a sense of presence. Some social contexts in which telecoms provide an acceptable substitute, and some contexts where they don't. Dividing the demand for telecoms/transport into different use categories.
Systems ThinkingCause and effect. Delayed effects - hysteresis.
Complexity. Lots of contradictory economic effects working simultaneously - very difficult to separate them out.