There are two flaws in this argument. First of all, highways are not always primarily an interstate concern. To be sure, the interstate highway system as a whole does connect the states. But at the margins, highway improvements often have a large local component. For example, suppose that Georgia transportation officials wish to create a new interchange for I-285, in Atlanta's inner suburbs, about 100 miles from the nearest state border. The people most affected by the interchange will not be out-of-state businesses, but the people who live near the interchange (who might have faster commutes if they are lucky) and developers who will wish to build near the interchange (assuming that the faster commutes make the interchange a more desirable destination).
Second, other forms of transportation have interstate benefits as well. The easiest possible example, of course, is public transit in a multistate metropolitan area, such as Washington's subway lines (which go from the District of Columbia to Maryland and Virginia) and Philadelphia's PATCO train (which goes from downtown Philadelphia to its New Jersey suburbs).
What about a more purely local service such as, say, a bus line in Jacksonville? The bus doesn't directly benefit Georgians- but then again, the new I-285 interchange doesn't directly benefit Floridians. And both may address national problems that ultimately affect everyone. For example, both the interchange advocates and the bus advocates will argue that their policies reduce greenhouse gas emissions and other forms of pollution, problems that are national and even international because pollutants travel across state and even national borders. The bus advocate will argue that the buses take cars off the road, while the interchange advocate will argue that by making traffic flow more smoothly, the interchange too reduces pollution. My point is not that either argument is right or wrong, but that both arguments address problems that are not limited by state borders.
What about a sidewalk or a bike path? Because these may cover such a narrow geographic area, these seem to be even less national than subsidies for cars and buses. But if the bus reduces pollution and congestion by taking a few cars off the road, so does the sidewalk and the bike path. And by encouraging exercise, the sidewalk and the bike path improve physical fitness and reduce obesity and the diseases it creates. Are these national problems? Probably not as much as pollution- but health problems too have national effects, affecting federal spending on Medicare and Medicaid, and affecting state and local spending as unhealthy people, like healthy ones, move across state lines.
I would not go so far as to argue that every transportation project has an equal interstate impact. But it does seem to me, however, that the line between "federal" and "local" transportation projects is not as bright a line as some commentators might think.