You may be restricted to locating any sensing devices within your own property boundary as I would expect that installing any equipment in, on or near a public thoroughfare may be frowned upon or perhaps even illegal in some regions, you may well get away with placing reflective photo electric beams across a roadway and mount the passive reflectors on a tree or post on the far side but the main beam units located within your boundary will still require power so maybe a small solar panel, charge controller, battery and an RF transmitter in a waterproof cabinet mounted alongside the beam RX/TX module would suffice in addition to an RF receiver back at the premises. I would be disregarding the road traffic altogether and focus on the boundary entry points alone, look at using Optex PE beams to monitor each individual point or even cover an entire front boundary including the driveways with a single long range set or stacked sets mounted on posts.
If the residence already incorporates a security system with a couple of spare zones then these may be utilised along with a dedicated output for a buzzer or siren, some systems do have additional macro features such as pulse count and trigger zones so the final configuration may be tweaked in programming to suite your requirements depending on the particular system if indeed any, reed switches on entry gates would be the simplest and most reliable form of sensing that someone is approaching the premises and using activity or motion detection on external CCTV cameras to trigger an audible alert is a waste of time in my view, the feature itself is generally best used to add either alarm or activity events to the DVRs log with time, date and camera numbers for rapid review as opposed to searching through hours of footage for a single event.
Cameras and DVRs that utilise motion or activity detection in an outdoor environment can and will be triggered on any movement within the designated detection grid, this includes animals, insects, rain, hail, moving trees and leaves, reflected light and shadows, you name it everything gets picked up and either logged or reported depending on the configuration and cameras that incorporate infra-red illuminators are the worst offenders as they readily attract moths and in turn cobwebs, spiders can be a huge annoyance and shifty at times.
As Ian recommended earlier a picture or diagram of the layout would assist immensely in addition to any information on existing systems that may be present, one final consideration is that vehicles leaving the site may drive other occupants mad with a buzzer or siren sounding frequently for no valid reason or occurrence.