The way to tell users how long it'll take to get to a location isn't the crow-flies distance, it's the actual travel time. There are various public APIs that expose this information, but their reliability and cost wouldn't be certain. Many provide free access for low-volume usage, to attract startups.
Otherwise, radius is a pretty poor indicator of travel time, because 10 minutes on a North-South motorway (or highway, as our American cousins call them) is a lot further than 10 minutes traversing around a city center. As the distance isn't that helpful, you shouldn't put so much weight on showing it precisely.
Much as JohnGB suggested, the best solution is to show results on a map, so that users can directly see the travel routes to each respective item, can see how 'brittle' the travel routes are (can distinguish places that are well connected and won't be unreachable if certain routes are closed). You may have to use aggregate pins at low zoom levels (perhaps with numbers that count the entries they collect.
Again, like JohnGB, I'd probably use a radius marker of some kind, but show results outside that radius (because elements just outside the radius will often be acceptable nevertheless, and the map will already be zoomed close to the circle). I'd shade the body of the circle slightly - though be careful, because you don't want to obscure the colours of the map's roads and symbols.
So, basically, this post is less an answer than an argument to go upvote JohnGB's post.