The 3M Performance 400 at Michigan International Speedway was one of the many races in the 2007 season that was delayed by rain. One of the terms heard on the race broadcast was 'weepers', as in "The track was dry, but then water started coming up from the weepers and the jet dryers had to start all over again". Weepers are places where water comes up through the track. Asphalt tracks aren't asphalt all the way through: The track starts with the native dirt, on top of which are placed layers of finely ground rocks and stones. If there's a lot of water, as there was this weekend in Michigan, some of the water can be retained underneath the track. That water has two choices: to drain downward, through the soil, or to move upward, through the asphalt.

Asphalt is very porous, so the stored water comes to the surface very slowly, which means that it takes forever to completely dry the track. Water can keep coming up for hours after the rain has stopped and the track surface has been dried.

There are two solutions. The first is to make it easier for the water to drain downward, say by creating reservoirs or other drainage structures and then making it easier for the water to go there than for it to go up through the track. The second is to make the track more resistant to collecting water in the first place. Developing "better" asphalt is not a simple task. The asphalt has to be extremely strong, it can't crack easily (including from weathering) and now we're going to demand that it's waterproof as well. The folks who re-paved Indy used polymerized asphalt (think asphalt with a rubbery binder) in the joints and made the asphalt as dense as possible by heavily compacting it.

One of the problems with weepers in the turns is that the water runs down the banking, toward the lower lanes and the infield. NASCAR changed their usual policy and told drivers to have all four wheels above the yellow line on the bottom of the track at all times because the lower areas of the track were more likely to be damp.

At least one driver would really like to see some research on eliminating weepers. The infield is a combination of concrete and grass. Concrete doesn't soak up much water, so the water collects in grassy areas. Jeff Gordon was spun near the end of the race and ended up mired in the infield mud. Even though his car was still running, he couldn't get enough traction to get it moving and back on the track. What normally would have been loosing a few positions turned into dropping from a top five to 27^th place.