Does a front always mean one air mass will be displaced by another? when is there no displacement?

Not necessarily, if the air masses are not strong enough, a stationary front is caused

Explanation:

A front is a block of separation between two air masses at different temperatures. They can be classified as cold, warm, stationary and occluded fronts. When a front is formed, a collision occurs between the two masses, generating a very dynamic activity, with electrical storms, gusts of wind and downpours.

A stationary front is one where neither of the two air masses are strong enough to replace the other. On such a front can generate atmospheric conditions such as prolonged rainfall and high cloudiness.

After several days, the stationary fronts may dissipate or become a cold or warm front. .

carla slowed down in the beginning of her trip, stopped, and then sped up