A Traffic Analyst is a role that is capable of checking to see whether a player can privately communicate. As an informative role, its night choice is to choose a player, and the analyst will learn whether or not there are any players that that player can legally communicate with outside the game thread. (The identity of the people that the target can communicate with is not learned, nor is the content of the communications.)
Note that merely having access to a private topic is not necessarily enough to be able to communicate; there will have to be a second living player in the private topic in question to communicate with. In general, the role will give a "can communicate" result on a player who shares a private topic with another living player, and also on a Mailman; and a "cannot communicate" result on anyone else.
A PT Cop checks whether a player has access to a private topic. Unlike the Traffic Analyst, this will get "has access" results even on players who are in a private topic by themselves, but will give a "no access" result on a Mailman (who has the ability to privately communicate, but without the use of a private topic).
Some Theme games have instead checked to see whether a player is actually making use of their private communication, i.e. checking to see if the player has privately sent a message toDay. This version of the role can be fooled simply by staying silent.
Both this role and the PT cop variant are considered Normal on mafiascum.net. The former should receive positive results only if there is more than one player alive in the private thread, the latter should receive positive results regardless. Additionally, this role's action should resolve after any kills.
Use and Power
Like a Gunsmith, this role gives unreliable information about a player's alignment; a "can communicate" result would most commonly come from the Mafia's factional communication, but could also be indicative of a Neighbor. One notable difference from a Gunsmith is that a Neighborizer effectively acts as a permanent Framer to a Traffic Analyst (meaning that it will effectively make the Traffic Analyst less and less useful over time).
The biggest difference from most other investigative roles, though, is that a Traffic Analyst becomes useless for finding scum once there's only one scum left; this makes the role useful for balancing even games with no special communication rules, as it has much less swing than a typical investigative role. As such, it's very much an early-game investigative role, compared to a Tracker which is more useful in the late game. Compare variant 2 of the Psychologist, which has similar properties.
Roles to check whether a player has access to a private topic have been seen sporadically in Theme games throughout the years. The first known use of the specific Traffic Analyst version was by callforjudgement in Micro 690.