Almost any argument against public transportation or better bike and walking infrastructure.

You can’t improve traffic by adding another lane. People who were using other means of transportation start to move over to cars, and then that lane fills up and you’re back where you started. This “induced demand” has been studied by traffic engineers for decades, and is very reliable.

You can improve traffic by offering viable alternatives to cars. Even if that means taking up a lane for bikes, buses, or rail. The induced demand effect works in reverse, too. People see they’re getting passed by the bus on their morning commute and make the obvious choice.