Do unreported crashes count toward the accident history warrant, or do only reported crashes count?

No, unreported crashes do not count. As an example, MUTCD Section 2B.07, specifically states " Five or more reported crashes in a 12-month period that are susceptible to correction by a multi-way stop installation. Such crashes include right-turn and left-turn collisions as well as right-angle collisions.” For multi-way stop signs rear-end collisions do not count because stop signs are not an effective way to reduce the amount or severity of such accidents.

Evidence of unreported crashes could be used in forming an engineering judgment, especially if they are combined with poor sight distance, skid marks, or other evidence of a safety problem. Carefully document such judgments as part of your risk management program.

Resources

MUTCD Section 2B.07

New York State Supplement to the Manual on Uniform Traffic Control Devices for Streets and Highways

February 2015