Air carrier Captain reported the approach end of PVU's Runway 31 is dangerously close to the departure end of SPK's active runway and presents a risk of midair collision.

Narrative

Experienced a traffic conflict (TCAS TA) due to another airport's traffic while circling to land in PVU. Commenced a VMC circle maneuver from the RNAV Runway 13; to land Runway 31. Winds required landing Runway 31; and the RNAV Visual Runway 31 was not an option due to marginal ceiling that would have prevented us from maintaining required visual contact and cloud clearance. While maneuvering base-to-final for Runway 31; we received a TA; and the target appeared to be within 2 miles (possibly closer) and less than 700 feet below us. Shortly after the TA sounded; we heard a traffic call on the other airport's CTAF; which we were monitoring in accordance with company chart recommendations. As we were in a very high workload moment; I don't recall how close the target ultimately became. We never acquired visual contact with the conflicting traffic. Already in a turn away from the conflicting traffic; we continued the circling maneuver to landing. Cause: Spanish Fork Municipal (SPK) lies less than 5 NM from the approach end of PVU's Runway 31; with a similarly-aligned runway. PVU's Class D airspace has a cutout for this airport; such that the only way to keep a circling maneuver within the Class D would be to fly less than a 2.5 NM final to Runway 31. Utilizing the protected circling radius of 4 NM; we were well outside of PVU's Class D during the maneuver; placing our base-to-final nearly over the departure end of SPK's active runway. The layout of these two airports; especially with one being uncontrolled; presents a unique risk of midair collision.Suggestions: For my part; I would have made a traffic call on SPK's CTAF during the circling maneuver; though doing so would need to be carefully balanced with the need to manage the flight path during a high workload situation. Having monitored the CTAF since well before commencing the approach; I would have hoped for a traffic call from the other aircraft before they were airborne; but it seemed that their first CTAF call was made after departing. Besides mutual CTAF communication; I'm unsure how to mitigate the risk of midair collision between these two airports; but there must be an in-depth discussion of how to do so. Of all the places I operate; landing on PVU Runway 31; especially when the RNAV Visual 31 is unavailable; presents a unique risk. Enough repeats of this flight's circumstances; in my view; will eventually lead to an accident.