Improvised explosive devices, the Afghanistan War's signature insurgent weapon, are now a quarter less effective than they were last year, according to Pentagon statistics provided to Danger Room.

During the year that just ended, from April 1, 2011 to March 31, 2012, insurgents detonated 7,166 bombs against U.S. troops and their allies. During the previous year – April 2010 through March 2011 – insurgents detonated 8,557 such bombs. They hit U.S. vehicles 816 times last year, down from 979 the previous year, and they hit dismounted foot patrols another 448 times, down from 598 a year earlier.

The bombs are also killing and injuring fewer troops. Pentagon data show that between April 2011 and March 2012, the homemade bombs killed 171 U.S. and coalition forces, and wounded 3,103 more. During the same period the previous year, the bombs killed 263 U.S. and coalition forces, and wounded 3,513 more.

But while the bombs are getting less effective, there are still more of them littering Afghanistan than ever before. Data provided by the Pentagon's bomb squad, known as the Joint Improvised Explosive Device Defeat Organization, or JIEDDO, show 16,069 insurgent bombs in Afghanistan between April 2011 and March 2012, which is a new high.

The silver lining: That's barely a 5 percent increase over the previous year's 15,767 bombs – coupled with a 25 percent drop in their effectiveness. Insurgents had raced to build bombs to counteract the Obama administration's troop surge: There were fewer than 9,000 homemade bombs in all of 2009.

The Department of Defense's bomb data is compartmentalized incomplete, and often confusing. The Pentagon's latest biannual report on the Afghanistan war, released on Tuesday, lists only the monthly totals of homemade bombs that explode – not, for instance, the total number of bombs; the number of bombs that U.S. and coalition troops spot before an explosion; or the casualty figures caused by those bombs. That report also breaks down data by month, while JIEDDO's data is quarterly. What's more, JIEDDO's data does not explicitly list successful bomb attacks, requiring reporters and analysts to derive those figures from the data the organization does provide.

JIEDDO did not provide an explanation for either the increase in the bombs or the drop in their effectiveness. The bomb squad, which is the target of much criticism on Capitol Hill over what its $20 billion budget has actually bought to protect troops, has sent a variety of different sensors into Afghanistan to combat the bombs, from tethered blimps to video-equipped drones to chemical-sniffing planesto dogs to six different kinds of ray gun. Additionally, the U.S. military has taken to unorthodox ways of protecting troops from bomb blasts, such as equipping them with Kevlar underwear.

The cheapness of the bombs practically ensures their proliferation. (Average cost to assemble in 2009: $265.) JIEDDO recently assessed that there are an average of 621 improvised explosive device detonations outside Afghanistan, a trend it expects to accelerate. Maybe the silver lining is that increasingly, insurgent bomb-makers get what they pay for.