> In particular, cars travelling at greater speeds are normally more spread out than slower cars, so that over a given stretch of road there are likely to be more cars in the slower lane

How many cars pass a sign over an hour-long period?

It should be equal to speed (miles/hour) X density (cars/mile) (say that speed and density are constant over the hour)

So more cars in the fast lane will pass that sign than cars in the slow lane will over the hour if speed (fast) / speed (slow) > density (slow) / density (fast)