As per the COLREG, A vessel constrained by her draught is a power-driven vessel which, because of her draught in relation to the available depth and width of the navigable water, is severely restricted in her ability to deviate from the course she is following,

What is important to note here is that the definition focuses not only on the available depth BUT ALSO the width of the navigable water. Assuming that the width of Traffic lane is not sufficient enough and surrounding areas are having shallow depths - will render the vessel to be severely restricted in her ability to deviate from the course. In this situation, the vessel shall in consultation with the VTS can display the lights that are displayed by the vessel constrained by her draught.