If your trying to implement some sort of security barrier/gate then I can add some lessons learned after i helped fix many systems that another company installed.
Infrared Light curtains and inductive loops seem to work the most reliably in the field for detecting vehicles (moving or stopped). Have integrated many of these fixes in automated gate systems at many different facilities. The advantage with light curtains was cost and 99 percent of the time solved our problems along with a single ir beam as a last ditch saftey method to prevent a 1000lb gate from slamming down on some one at 30mph.
We used a micro controller to monitor the light curtains and inductive loops and the firmware was modified for each unique situation. Loop and light curtain placement was usually different at each location, so timings had to be adjusted to make a system work properly.
Physical location of light curtains (side looking vs top down) to get anti tailgating requirement work. Especially for pedestrians and motorcycles/bikes
Infrared light curtains were very effective at stopping tailgating when combined with in ground inductive loop sensors. (its all about the physical location and timing of each component in relation to each other)
Light curtain sensors must be as low to the ground as possible and account for the largest vehicle height possible (Vehicles with trailers can cause some interesting consequences when the 1000lb barrier arm comes down because the system thinks someone is tailgating!
Make sure you really understand the requirements. (every possible situation that can happen if possible). When a trailer hitch is lower than your light curtain the system sees a gap between the truck and the trailer and thinks the trailer is tail gaiting. Not good
. Good thing the customer defined the height requirement
Inductive loop sensor placement is key, since sensitivity adjustment is degraded if they are too close together. Changing the number of wraps in your detection loop and specs of the loop amplifier/receiver hardware is also a variable. Also be ware of existing metal in the area before loop placement and take appropriate measurements before install (mock up loops and take inductance measurements onsite). Man hole covers or buried metal pipes can cause problems
Cameras with recognition software were used in some installs, but cost can get prohibitive, and complexity goes way up. (network cable, hardware, expensive enclosures). Plus you have to deal with the notification process of what to do with the information if it triggers some kind of security issue. If its used to just close the gate then your good, but usually someone wants to know whats going on and under what conditions can normal operation resume.
These are just a few things that come to mind.