To get alerts only when an event matches a specific query, just pin the query.
- Perform a search like "people approaching walkway" so that the query is in the search history dropdown, then
- Pin the query in your search history shown when you click inside the search box.
As an example, if I want to get alerts when a car approaches my driveway, I can do a search for “car approaching driveway”:
Once I've used that query to search, it's in my search history:
I then pin the query by pressing the thumbtack icon, which turns black:
To turn alerts off without unpinning your queries, see How do I turn alerts on/off.