You could use motion alerts, via Webhook, to trigger the capture of footage:
https://documentation.meraki.com/MV/Advanced_Configuration/Motion_Alerts#Alert_Only_on_People_Events
https://developer.cisco.com/meraki/api-v1/#!get-device-camera-video-link
There's a beta tool available to trigger such alerts only on people detection too. Right now I don't believe that extends to vehicles. Bear in mind that if you also triggered the capture of screen grabs, you could send the output to AWS Rekognition (for example) which could detect vehicles, people, text and all sorts of other things.
You may also find MV sense particularly useful in pre-processing and adding more granular information: https://meraki.cisco.com/lib/pdf/meraki_datasheet_mv_sense.pdf