However, Mill may respond: "This would deny people jobs who clean up freeways."
Kant's categorical imperative states that the morality of an action can be determined by asking the question: if everyone did what I am considering to do, what would happen? If the answer is that the world would be a better place, then the action would be morally right to do. If the answer is negative, then one should refrain from doing it. Kant believed in the universality of certain actions or behaviors. He did not believe in situational ethics or relativity. An action is right or wrong in and of itself. He would argue that if everyone threw trash out the car window, then the world would be a worse place; therefore, the action is morally unacceptable for everyone.
Mill, though, was a utilitarian. He believed that the morality of an action could be determined by whom it benefits: whatever benefits the greatest number of people. He argues by effect. So Mill would argue that some people may be denied jobs if no one threw trash out the car windows. Mill would argue that the action might at times be acceptable.
However, in this case, I think Kant wins. If everyone, or even if many people, threw trash out the car window, the landscape would be pretty ugly, and there are not that many who are willing to pick up this trash--for less than minimum wage. So the number of people who would profit would be few if the majority of people threw out trash; and the number of people who would suffer from viewing ugly landscapes or enduring unsanitary conditions or having their own properties sullied would be many. So, Mill would lose with his own theory.
Kant wins with his theory and with Mill's.