detecting passing vehicles
Something that is becoming very popular in europe is the ANPR system.
http://en.wikipedia.org/wiki/Automatic_number_plate_recognition

There is various software available out there to handle a video stream/ pictures to recognize the plates from hundreds of typical number plate fonts.

Best camera to get is a wireless night mode (Infrared Emmitors) As number plates react well on IR beams to reflect the background and adsorbs on the letters. You connecto the WIFI cam to your local wireless N network (good coverage outside- up to 50metres! and good for streaming 640x480 or even 720p video now a days!) and have a computer/notebook/netbook run software that collects number plate data. 

With this data you can determine frequency of each car. So you could see that car A drives on your road 4 times a day. rough 8am  then 1pm then 1.30pm and lastly 5.45pm. You can get condendesed data and see peak times and dip times.

You will have to consult you local council/muunicipality/governing body on the law of cameras in a public place. Gathering ONLY number plates numbers will be ok because you are gathering statistical data and not asscotiating these figures with any personal details...e etc etc

You can use this one using a Video Transmitter

http://www.maplin.co.uk/40m-high-resolution-night-vision-outdoor-cctv-camera-351984?c=froogle&amp;u=351984&amp;t=module

Or this one using a IP Network Router
http://www.henrys-electronics.co.uk/shop/Wifi_IP_CCTV_Cameras.html

http://www.bestofferbuy.com/IP-Wireless-WIFI/LAN-300K-CMOS-Waterproof-Camera-with-40-LED-IR-Night-Vision-p-50682.html?currency=GBP&amp;utm_source=gbase&amp;utm_medium=cse&amp;utm_campaign=gbase

BUT YOU WANT THIS BABY!

http://www.cctv42.co.uk/content/124/anpr-camera-with-powerful-9-22mm-zoom-lens-adjustable-ir-lighting-and-osd

Its not wireless but its not 1000pounds either. some compromise can be made on all the cameras

The big boys who deal with stuff

http://www.automaticnumberplaterecognition.co.uk/anpr-products.html

Some OpenSoftware

http://www.mp3car.com/software-and-software-development/124529-automatic-number-plate-recognition-anpr.html

http://www.cctv-information.co.uk/i/An_Introduction_to_ANPR

Good luck :)