basically aim a camera out the window and calculate speed of cars on
the street.

Did a little research and found some work a couple Cambridge (UK) grads did 
with portions of openCV on their rPi
https://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/image-processing/intro.html 
I had the files downloaded, untarred and maked pretty much just like the 
getting started instructions*. Plugged in $5 webcam, compiled C example
programs and they just worked*. Framework on first try- nice. Using
an rPi2 demonstrated about 10fps (640x480) to hdmi image on desktop,
even got 8fps with streaming edge detection, less than five pic/sec
over lan.

My program grabs
three images in consecutive lines of C and the images diffed – if
the first two (depends on direction) images are similar along a
horizontal line the length of the first to last image miscompare is
recorded and is proportional to speed.




Worked for me -
*Don’t wget or sudo tar commands
*c programs need <stdlib.h> and add -lm to gcc command line.