The quality of the bombs has been declining, as the talent pool of bomb makers and placers is diluted by more demand for their services, and increasing casualties caused by American and Iraqi counter-terrorism efforts. The larger number of bombs is due to two factors. One, more American troops are operating in the Sunni Arab heartland, especially Anbar province (western Iraq). These are the last refuges for the diehard Sunni Arab supporters of secular or religious dictatorship in Iraq. While over a quarter of the Sunni Arab population has fled the country, those that remain recognize that the roadside bomb is still their most effective weapon. Getting bombs made and planted costs money (the bomb makers and planters offer their services to the highest bidder), and the diehard Sunni Arabs are spending like there's no tomorrow. For them, there is no tomorrow if the government gains control over all of Anbar, so it's a matter of use it or lose it.
For Americans operating in Anbar, there's a lot more road space to check for bombs. UAVs are constantly patrolling roads the troops use, as are teams of troops equipped with night vision equipment and other sensors, and a direct line to smart bomb equipped aircraft overhead. But the (cash) incentives for getting a bomb to damage American vehicles was so great, that there are still Sunni Arabs willing to try. But it's becoming a lot more dangerous. The bombs are not as well made, with wires replacing wireless detonation because of the success of American jammers. Using a wire makes it more dangerous for the crew who sets off the bomb, and more of them are getting wiped out. Unlike Baghdad, which is largely patrolled by Iraqis now, in the wide open spaces of Anbar, if the Americans spot you, they will usually get you.
Iraqi terrorists are increasing their use of roadside bombs, but having less success with them. Currently (as of July 2006), 2,625 roadside bombs were used, up from 1,454 in January. There are several other interesting trends accompanying this increased use of bombs. Fewer of them are being used against American troops. Now, twenty percent are being used against Iraqi soldiers and police, compared to nine percent in 2005. Iraqi civilians are hit by ten percent of the bombs now, compared to five percent last year. The increased number of bombs used against Americans are having less impact, with American casualties from roadside bombs declining.