If public roads are public goods, in what way are they non-rivalrous? I get why, say, the military is a public good, but I don't get why public roads are public goods, since they can't be non-rivalrous, given that at any time only a subset of the population can actually be on them.
The property of rivalry is a continuous (rather than binary) variable.*

A good is rivalrous if my consumption of it reduces the amount that can be consumed by others.

So, a particular Big Mac is fully rivalrous, because each bite I take from it reduces (by that exact same amount I've bitten) the amount left for you.

The degree to which roads are rivalrous is lower: My driving on a deserted road at night does not reduce at all the amount that can be driven on the same road by others.

But your doubt is correct—it is not generally $0$: My driving at peak hour at a city center does reduce the amount that can be driven by others.



Ideas are a good (and possibly sole) example of a good that's perfectly non-rivalrous. (My "consumption" of the Pythagorean Theorem leaves no less for anyone else.) Side point: But with the right laws and accompanying enforcement—e.g. patents, copyright—even ideas can be made excludable.

Defense is often given as a classic example of non-rivalry. But I would argue that although defense exhibits a very high degree of non-rivalry, it is less than perfectly non-rivalrous. (The resources used to defend John in New York from a nuclear attack does reduce the US Department of Defense's ability to defend Jane in Hawaii. The resources used to defend a nation with over 1B people are greater than the resources used to defend a nation with less than 1M people.)



*Likewise, the properties of excludability and being a public good are continuous (rather than binary) variables. (Many introductory textbooks tend not to emphasize this important point sufficiently.)