The impact of public infrastructure on economic growth was studied in 48 countries from 1960 to 2001. The researchers found that having more public capital positively affects economic growth, but the relationship is not linear - it's stronger at lower levels. This effect was consistent across different time periods and depreciation rates. Interestingly, in countries outside the OECD, the positive impact of public capital on growth becomes even more significant when looking at longer time periods.