What I would do is make a new layer, take a fuzzy brush and go around the tires and a little bit behind with a light grey, add a fair bit of noise to it but not overwhelming, and then add directional blur to match the direction of the road. Last step is lower the opacity to about 90-95%.
My first attempt at doing a rain edit! It may not be as good as some of the other pieces of work in this thread,
but at least I gave it a good go. Click for Lightbox
Version 2:
I took in the feedback that leeislee gave and decided to make some tweaks to the image.
Original:
Brilliant. you've avoided the mistake of adding a blanket sheet of rain. When a car is moving of even when a person is walking you don't really see a sheet of rain, you see a bit of it and the rest is almost invisible unless it's a complete torrential rain storm!
don't forget everyone, rain never falls 100% in one direction. slight variation in rain angle and transparency will add more realism to your rain edits....oh, and desaturate the colours abit too!
Great work in this thread though...some really good work going on!
Also, if you have such a wet road and strong rain, you would see water sputtering from the road and car. I know that's hard to add though and a lot of work to get it right.
I might do a video tutorial soon. I'm just not sure if this wouldn't be kinda pathetic considering how many better artists are out there.