The number of roadside bombs being planted is falling sharply. In the Baghdad areas, for example, there were only 18 of them in the first week of March, compared to 40 a week in November. There are two main reasons for this. One is the greater use of ground and air patrols of the roads where the bombs are most often placed. The silent UAVs are particularly effective at catching Iraqis planting the bombs. The bombers are then usually caught or killed. This has a deterrent effect. Another deterrent is the percentage of bombs already planted that are discovered before they can detonate. This has done up to 60 percent, from 48 percent last November. Another factor is the growing number of Iraqi resistance groups that are being uncovered and their members arrested. There is not an unlimited number of these groups, and they are being taken down at an increasing rate.