The OP states that a queue is worse than a congestion, and the phrase queue ahead/after xxx expresses greater certainty than the terms caution and likely.
Dictionary.com defines a queue and a congestion as
queue: a line of people, vehicles, etc, waiting for something: a queue at the theatre
1. the state of being overcrowded, esp with with traffic or people
e.g., The city has become gridlocked, with rush-hours replaced by semi-permanent congestion
Oxford Dictionaries Online says
queue chiefly British A line or sequence of people or vehicles awaiting their turn to be attended to or to proceed
e.g., Vehicles previously stuck in queues past the A2 junction suddenly speed up and try to get the best position as three lanes expand to eight for the toll booths.
congestion The state of being congested
e.g., The bus lanes are designed to ease traffic congestion but they remain controversial
But that depends where your focus lies. If it is speed, then yes, a sign informing motorists that there will be queues ahead will almost certainly mean that drivers should expect significant delays. Traffic will move at a much slower pace. A sign informing about possible congestion ahead will mean that traffic will be much slower than normal but it is less likely that drivers will be at a standstill.
However if your focus is of safety, then I would argue, signs warning of congestion ahead should be taken more seriously, motorists would do well to employ defensive driving measures. Congested traffic is more likely to lead to road accidents because of a "Higher chance of collisions due to tight spacing and constant stopping-and-going"
image: European Approach to Congestion Management
Source: Traffic England