People often describe routing as finding the shortest path. In field service it is almost never that. It is finding the shortest path that is still allowed.
Why it matters
A route ignoring constraints is cheap and useless: it sends an unlicensed technician to a termite job, or arrives at a restaurant during lunch service. Every real constraint narrows the set of possible routes, which is why adding them makes the result longer — and why removing a constraint that was never real is the single cheapest improvement available.
How you notice it slipping
Constraints live in people’s heads rather than in the system, so the plan is wrong until a human corrects it each morning. Or the opposite: constraints have accumulated for years and nobody can say which still apply, so the planner is solving a problem stricter than reality.
What to do
List them and mark each as hard or soft. A hard constraint is one where breaking it is a failure — a licence, a legal access limit. Everything else is a preference with a price. Operations that have never made that list are usually carrying several expensive constraints that no customer ever asked for.