The optimism bias on the overall project duration (from the gestation period through to the implementation stage) was also measured. The project duration overruns will be caused not only by delays during the construction of works but also by delays during the procurement of the project (i.e. prior to commencement of construction). The project duration optimism bias is highly dependent on the life-cycle stage at which the business case information is obtained (i.e. strategic outline case, outline BC or full BC) as a proportionately large amount of time may have passed between these stages. In addition, the length of the gestation period could be greater than 10 years resulting in unrealistically small project duration optimism bias. Therefore this paper does not present the results nor give guidance for project duration optimism bias.