Accident migration refers to a situation where action to reduce road traffic accidents in one place may result in those accidents resurfacing elsewhere.
For example, an accident blackspot may occur at a dangerous bend. The treatment for this may be to increase signage, post an advisory speed limit, apply a high-friction road surface, add crash barriers or any one of a number of other visible interventions.
The immediate result may be to reduce collisions at the bend, but the subconscious relaxation on leaving the "dangerous" bend may cause drivers to act with fractionally less care on the rest of the road, resulting in an increase in collisions elsewhere on the road, and no overall improvement over the area.
In the same way, increasing familiarity with the treated area will often result in a reduction over time to the previous level of care (regression to the mean) and may result in faster speeds around the bend due to perceived increased safety (risk compensation).