That's true, it's different for shippers. They should be taking added security measures. It is easy to get a list of the public relays, and there are only about 800 entry guards, so LE could contact the main ISPs of that city and ask for all users who connect to those IP addresses. The density of Tor users is fairly low, so it would be a short list. Getting all the bridges is a lot harder, but not impossible. Bridges could still be useful by making LE's job harder. A VPN also might help in that situation, but you would want to get one outside of your country. A private, unpublished bridge with obfuscation would be the safest. Rent some VPSes in Iceland or Malaysia and set up bridges, LE won't know to look at those. I mean, connections to those IPs will be lost in the tens of thousands of other IP addresses that users in that city connect to on a daily basis.