Image processing library:
OpenCV or if you have Matlab with the image processing toolbox it would be very nice.
Algorithm wise
Counting cars on a road is not as simple as it appears. Lets suppose they have a invariant scale (say 1.0).
- Create yourself a Hough transform algo to vote for rectangles with about the same width and height as your cars.
- In your vote buffer find the 'n' greatest vote scores and this is your number of cars.
This is not so complicated and OpenCV can be pretty useful. You can estimate the maximum value of your hough score for each car so you can set thresholds to skip outliers.