hat the size of the effect is not as large as that estimated by earlier studies [such as those by Aschauer (1989)]. Based on samples of several OECD countries and broken down according to economic sectors, the findings indicate that the efficiency impacts of infrastructure tend to be positive - but relatively modest - in almost all sectors. INFRASTRUCTURE TO 2030: TELECOM, LAND TRANSPORT, WATER AND ELECTRICITY - ISBN 92-64-02398-4 © OECD 2006 1. A CROSS-SECTORAL PERSPECTIVE ON THE DEVELOPMENT OF GLOBAL INFRASTRUCTURES TO 2030 What is important to note is that the returns on infrastructure investment take time to materialise. The more recent studies indicate that the long-term impact of infrastructure on the economy is positive, and that while an increase in investment in public capital acts as a substitute for private capital initially, in the long run the dominant effect is one of complementarity. However, it would appear that investment in infrastructure does not, on the whole, create (directly) long-term employment. The studies also suggest that patterns of underinvestment in infrastructure in some countries may have something to do with the difficulties governments experience in estimating the overall long-term effects of infrastructure on the economy. Making the "right" decision regarding infrastructure development is often difficult because of the public good nature of the benefits (how much is enough, who should benefit). Moreover, the broader impact of infrastructure is clearly conditional on how efficiently it is used. Poorly managed or poorly conceived infrastructure does not necessarily generate the same return. The productive impact of an infrastructure depends not only on the magnitude of the investment, the project's design and efficient management, but also on the nature of the investment and its integration into an existing set of infrastructures, i.e. how it improves the network. Thus, first infrastructures only have limited impact on private sector productivity since their effect is primarily lo