The study setting was the 196-km-long Karachi-Hala road section (km 16 to km 212 from Karachi centre), for which the three RTI databases were available. This is a four-lane highway, two lanes in each direction . The lanes are separated by a ground surface, but there are no physical barriers. Traffic counts vary between 16 356 to 24 707 vehicles per day on this section . These high traffic counts are related to the economic activity in Karachi, the most populous city of Pakistan, accounting for 70% of government's trade and industry-related revenue . In this retrospective study, characteristics, such as outcome and user category, of traffic injury patients reported to highway police, ambulance service, and hospital ED from January to December 2008 were compared among the three databases. Data on crash characteristics were too scarce to be compared.
A crash was defined as any event where a motorized vehicle, including motorcycles, was involved in a collision with another vehicle, road user, or other obstacle, and reported in either of the police, ambulance, and hospital ED datasets [13, 15]. RTI was defined as any person incurring a physical injury as a result of a crash reported to any of the above datasets [13, 15].
Since 2004, the National Highway & Motorway Police (NHMP) has been enforcing traffic rules on this road section. Administratively, this section is considered as Sector I of South-Zone of NHMP and is divided further in four 46 to 51 km-long beats: beat 35 (km 16 to 62 km), beat 34 (63 to 114 km), beat 33 (115 to 162 km), and beat 32 (163 to 212 km). NHMP deploys on each beat four motor vehicles and four patrolling officers per eight-hour shift .
For every crash, a standard accident analysis report is filed during the first 24 hours by the attending NHMP officer . A copy of this report is kept in the NHMP regional office. Details on the crash and those involved are recorded on a separate accident register. From these reports and registers, information was extracted on time, date, location of crash, and whether it was fatal, involved injury, or was without injury. We also extracted information on name, age, gender, outcome (dead; transported to hospital; and not transported to hospital), and, if transported, name of the hospital.
Ambulance records were obtained from Edhi Ambulance Service (EAS) logbooks. EAS is the largest private philanthropic ambulance service in the world . Since 1973, the EAS has been progressively increasing its ambulance posts from main Pakistani cities to the important highways in Pakistan [22, 23]. For transporting injured patients, EAS has established six ambulance posts, mostly near main towns on Karachi-Hala road section: 1) Sohrab Goth (12 km from Karachi centre), 2) Karachi toll plaza (km 28), 3) Edhi centre (km 56), 4) Nooriabad (km 94), 5) Hala Naka (km 160), and 6) Hala city (km 212). This service is freely available to injured patients, and funds are raised by transporting other patients. In most cases, ambulances are only staffed by the driver. A clerk at the post can come with the driver if he thinks this is justified, for instance, crashes with multiple patients. The ambulance communicates with the emergency post through a wireless system or cell phone.
RTI patients or bystanders can contact EAS using the free emergency-access number 115, which connects them to the main city centre . Information is then transmitted by wireless or cell phone to nearby posts, which finally dispatches the ambulance(s). When reaching the scene, attendants separate injured from dead patients. Those severely injured are transported to the nearest hospital; preference is given to the government hospital if available. All information on the intervention, including crash location, injured patients identity and outcome, is then transmitted by wireless or telephone to the regional centre, which records the information in a central log book. We photocopied these log books from the regional centre at Karachi. Crash details such as date, time, location, and whether it was fatal or involved injury were extracted from these books. Similarly, road user details such as name, gender, age, user type (pedestrian, motorcycle rider, or vehicle occupant), and outcome (died, including whether the person died at crash scene, during transport, or at hospital ED; injured and transported, including hospital taken to; injured and not transported) were extracted from these log books .
The Road Traffic Injury Research & Prevention Centre (RTIRP) at the Jinnah Post Graduate Medical Centre (JPMC) has systematically collected standard forms since September 2006 , information on RTI patients presenting at the hospital ED of the five largest teaching hospitals in Karachi: 1) JPMC, 2) Abbasi Shaheed Hospital, 3) Civil Hospital Karachi, 4) Liaqat National Hospital, and 5) The Aga Khan University Hospital. Details on their data collection methods are described elsewhere [24, 25].
This dataset includes information on the crash date, time, and location, and patient's name, age, gender, road user type (pedestrian, motorcycle rider, or vehicle occupant). Further information on whether the patient was wearing a helmet or seat belt was available. The New Injury Severity Scores (NISS, range 1 to 75) , and outcome (discharged, admitted/referred, or died) of patients were recorded during their stay in the hospital ED. Information on RTI patients involved in crashes on selected road section was extracted from this dataset.
All information was recorded in Excel® spreadsheets. We compared percentages for crash and injury patient characteristics across the three datasets. For the hospital ED dataset, we described outcome for the following NISS categories: minor injury, scores ranging from 1 to 3; major injury, scores ranging from 4 to 8; and severe injury, scores above 9 . Same records present in two or more datasets were matched using crash date and time, name, age, and gender of RTI patients. For matched records, we identified differences in reported outcome. To estimate total fatalities, a person reported injured in police statistics, but dead in ambulance data was considered as dead. The number of unique deaths and injuries were then assessed after removing duplicates of records appearing in two or more datasets. Ascertainment rate for police, ambulance, and hospital ED records, as compared to these total fatalities and injuries, were computed . Capture-recapture methods were not used to estimate road burden, because RTIs away from Karachi might not have the same probability of being captured in the hospital ED dataset, thus violating one of the basic assumptions of the method . The unique records and traffic counts from highway authority were used to compute overall traffic fatality and injury rates per vehicle kilometre in 2008 for this road section . Considering that there would be missing information for variables used in linking datasets, we carried a secondary analysis considering situations where at least one of the variables could be matched.
All the police, ambulance, and hospital ED data used in this study were publicly accessible and analyses were conducted with approval from the respective institutions. Furthermore, this manuscript did not permit identification of any RTI patient.