As motorists in northwest Detroit can tell you, exiting the Southfield Freeway for Seven Mile offers little relief for weary suspensions: a series of freeway potholes -- one the size of a kitchen sink -- gives way to an unrelenting legion of smaller holes and bumps. And while the state's multibillion-dollar effort to rebuild its thoroughfares over the past 15 years has helped reduce potholes, county road officials agree, many problem areas remain. They can also be costly. The average pothole claim for Michigan AAA members was $638 in January and February, up from $624 in 2006, said spokeswoman Nancy Cain.