Traffic volume and crashes and how crash and road characteristics affect their relationship – A meta-analysis

•Meta-analysis of the relationship between traffic volume and crash numbers.•Crashes increase with increasing volumes but mostly at a lower rate.•The relationship is strongest for multi vehicle crashes, at high volumes, and on freeways.•The relationship is weaker for fatal than for less serious crashes.•Crash prediction models may improve from disaggregation by crash and road type and volume level.