The laser and primary camera usually face forward.
The secondary camera system is tied into the primary, and can provide photo (or video?) of the car or bike after it passes, so cars can still have the front plate on the primary photo.
If they set up the camera facing rear then there is no secondary unit.
Primary unit with flash:
http://members.iinet.net.au/~macromr...iScanspeed.jpg
Inside Primary unit:
http://www.atzonline.de/cms/images/v01-10-02.jpg
Secondary (rear facing) optional unit:
http://l.yimg.com/fv/xp/wan/20100419...m7t7DIanLEIA--