Rainway

It's finally raining a little bit, here in this drought-land that I live in by the sea. But the funny thing is that you don't notice the rain much except in certain places. Or should I say at certain speeds? There is no problem with the barely noticeable moisture in the air, until you get up to about 55mph on a freeway. Then, suddenly, you need the wipers. So my question is, are the clouds more dense over the freeway? Or am I just hitting more drops more quickly at that speed?