And choosing another site for comparison isn't easy: Choose one too close to intersections with cameras and it could experience a so-called spillover effect, when camera-less intersections along the same route are affected by motorists conditioned by cameras.

WSJ: Hard to Put Red-Light Violations Under a Lens

In other filed comments, the Spark Institute, which represents companies that service 62 million plan participants, warns that the department's questions are so far reaching that they could have a negative spillover effect on existing third-party advice programs.

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