► Lethality of collisions between whales and ships increases with speed and can be mitigated by slowing ships. ► Understanding the conservation benefit of specific speed limits depends on knowledge of the status quo speeds used by the ships being managed. ► We used AIS data of the actual speed of transiting ships to model the lethal risk reduction that could be expected by implementing restrictions. ► The choice of speed restriction had a major impact on lethality to whales: 16 knots = -3.7%, 14 knots = -11%, 12 knots = -29.4%, 10 knots = -56.7%. ► Results provide insights to reduce lethal collision risk to whales with methods applicable to other areas. Collision with ships is a significant cause of mortality among endangered whales. Collision lethality increases with vessel speed and mitigation includes slowing ships in whale dense areas. The 2181 km 2 Stellwagen Bank National Marine Sanctuary (SBNMS) is a site of numerous whale/ship collisions. To understand how speed reduction measures reduce lethal collisions, we used GIS to apply hypothetical speed reductions to observed ship traffic within SBNMS. During 2006, we collected complete AIS data from SBNMS vessel traffic. We created 1.85 km 2 ( N = 810) grid cells covering SBNMS and determined each cell’s predicted probability of lethality (PLETH) from the cell’s mean speed and a mortality curve. We calculated average PLETH for the entire sanctuary (SPLETH), and used SPLETH to index status quo risk. We applied speed limits of 16, 14, 12, and 10 knots on transits and recalculated SPLETH for each scenario. Our analysis included 2,079,867 AIS points to derive 74,638 cell transits by 502 ships (>295 t). Sanctuary mean ship speed, by cell transit, was 13.5 knots (SD4.3, range 0.1–42.2). The choice of speed restriction had a major impact on SPLETH: 16 knots = −3.7%, 14 knots = −11%, 12 knots = −29.4%, 10 knots = −56.7%. The conservation benefit of speed restrictions is influenced by the status quo speed of ships from which risk must be reduced. As most areas lack such data our results can provide managers with a better understanding of how speed restrictions might reduce risk in their waters.