A former Arup traffic forecaster has conceded that it was "utterly absurd" to devise traffic models for Brisbane's Airport Link that assumed drivers would think the tollroad was cheaper than free roads because they paid tolls electronically.

PPB Advisory, the receivers for Airport Link's former owner BrisConnections, is suing international engineering firm Arup in the Federal Court, claiming it made misleading and deceptive forecasts and should pay for the company's losses, estimated at $2.2 billion.