The study aimed to (1) assess the magnitude of road traffic injuries in a country missing a formal linkage system of police with hospital data, (2) quantify the under-reporting, and (3) produce a convenient algorithm exploring its constituent components. Linkage of disaggregate (individual) data collected by the Road Traffic Police (RTP) with those by the Emergency Department Injury Surveillance System (EDISS) on the Greek island of Corfu and coded with different classification systems was carried out. The applied four-step methodology, also comprising the calculation of under-reporting coefficients of the variation by basic demographic variables, mode of transport and injury outcome, led to the identification of the overall under-reporting from either registry. RTP data captured 96.6% (coefficient: 1.035), whereas EDISS only 54.4% of total fatalities (overall concordance: 51.1%). On the contrary, EDISS captured 94.6% of non fatal injuries, whereas RTP only 16% (coefficient: 6.238), resulting in a low overall concordance (10.6%). Considering severity of injury assessed by EDISS, by using the ISS, as the gold standard, RTP data misclassified 20.3% of severe injuries as less severe, whereas a statistically significant difference in the under-reporting by gender was also noted. In conclusion, relatively simple methodologies can provide essential coefficients to assess the actual numbers, severity and components of road casualties by complementing routinely collected RTP with sentinel Emergency Department reporting systems.