A hazardous event was assessed as severity S3, exposure E4 and controllability C3, giving ASIL D. If, for the same event, only controllability were C2 (most drivers can normally avoid the hazard), what would the ASIL be?

Answer: C. ASIL C

In the ISO 26262 ASIL table, only the combination S3, E4, C3 gives ASIL D; lowering any one of S, E or C by one level lowers the ASIL by one level to ASIL C. As a handy rule of thumb, if the sum of the S number (1–3), E number (1–4) and C number (1–3) is 10 it is D, 9 is C, 8 is B, 7 is A, and anything lower is QM. In this example, 3 + 4 + 2 = 9, so it is ASIL C. It neither stays at ASIL D nor drops two levels.