I would try a narrow beam US sensor pair, this model would probably be best... your accuracy will still be off though...http://www.maxbotix.com/MB1240__XL-MaxSonar-EZ4.html
Any better ideas for a speed sensor? My idea would cost me well under 10$ so price is also a part in this.
Not if it's low enough on the ground to be below the windshield level, just low enough to hit the body (as in, just above the bottom of the car's body), then there is no way that would interfere with the driver... plus, the laser's ray would be invisible to anyone driving by unless it directly hits them in the eye (which it won't), so no one would probably notice the fact that their speed is being checked unless they take the time to mess around with the four suspicious boxes placed beside the road (you should disguise them as mailboxes )
Any hints for a suitable solar panel and how that would work?