White or grey? At night (dawn, dusk, or anytime lights are from behind) any smoke is more noticed.
White smoke is either unburned fuel that has been injected, atomized, but not burned (unlikely), or it's water (headgasket leak, atmospheric moisture) also unlikely to only appear on the highway.
Light grey particulate, especially under the circumstances you describe, is common and almost normal. What I suspect you are seeing is the coating of exhaust soot on the exhaust system walls being blown off by the increase in exhaust gas flow and volume, as well as a richer fuel to air mix adding a bit more fresh soot.
It comes down to your tolerance for following "the golden rule": Is there enough of a cloud being left behind that you, if you were the following driver, would be annoyed?