Fatality risk and change in velocity from head-on crashes involving passenger vehicles: a systematic review and meta-analysis

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OBJECTIVE: Head-on crashes on undivided roads are particularly dangerous, often occurring at high impact speeds that may lead to fatal injury. This study examines the relationship between changes in vehicle velocity (delta-V) and fatality risk for head-on passenger vehicle crashes, which may assist regulators in setting safer speed limits for undivided roads. METHODS: A systematic review and meta-analysis was conducted to estimate the fatality risk by delta-V for head-on passenger vehicle crashes. Studies were included if they reported delta-V and occupant survival, and excluded unbelted occupants, crashes with multiple impacts or vehicle rollover, or simulated data. Random effects logistic regression with study-level random effects was used to estimate the fatality risk by delta-V, the change in fatality odds, and the delta-V corresponding to 1%, 5%, and 10% fatality risk. RESULTS: Five data sources met inclusion criteria including two published studies, two publicly available datasets, and a government dataset. The data included 6,247 casualties and 185 fatalities. The final random effects logistic regression model estimated a 6.0% increase in fatality odds for a 1 km/h increase in delta-V (OR: 1.06, 95% CI: 1.05, 1.07) with a 1% fatality risk at 23.3 km/h (95% CI: 14.2, 32.3), 5% fatality risk at 51.3 km/h (95% CI: 42.8, 59.8), and a 10% fatality risk at 64.0 km/h (95% CI: 55.4, 72.7). The fatality risk varied by region of data collection and more recent data estimates a similar increase in the odds of fatality for a 1 km/h increase in delta-V but an increase in delta-V for a 5% fatality risk (59.7 km/h, 95% CI: 51.0, 68.4). Similarly, there was an increase in delta-V for a 1% fatality risk (29.5 km/h, 95% CI: 18.5, 40.5) and for a 10% fatality risk (73.4 km/h, 95% CI: 63.5, 83.2). CONCLUSIONS: The survivability in a head-on crash has improved over time. The most recent data supports setting speed limits no higher than 60-70 km/h for a 5% fatality risk or 30-40 km/h for a 1% fatality risk on undivided roads which do not have a median road safety barrier installed to minimize the risk in a head-on crash fatality. For jurisdictions willing to accept a 10% fatality risk, the results suggest setting speed limits no higher than 70-80 km/h. The observed improvements in fatality risk may be due to recent advances in vehicle technologies, and some consideration should be given to low-to-middle income countries (LMIC) where such technologies may not be prevalent in their vehicle fleets.

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