Culpability analysis is still a valuable technique.

The February 2013 issue of the IJE included our study which used culpability analysis to examine the association of cellphone use with motor vehicle crashes. In a commentary in the same issue, Sanghavi stated that ‘culpability analysis won’t help us understand crash risk due to cellphones’. Sanghavi’s categorical rejection of culpability analysis is unfortunate and seems based on a misunderstanding of the method and perhaps aversion to the term ‘culpability’. Culpability analysis, also referred to as responsibility analysis, is a powerful technique with a long history in road safety research. The method was first used in 1951 by Smith and Popham to study the association of alcohol with crashes and has since been applied numerous times to the study of crash risk. A standard feature of the culpability design is that all drivers have been involved in a crash and are therefore subject to standard legal or medical investigations which may reveal the presence of potential risk factors for crashing. As such, culpability analysis is well suited to the study of transient exposures, as related to crash risk, that are typically difficult to determine in a valid control population of non-crash involved drivers. For example, when studying drug use in relation to driving, it is a challenge to measure the actual rate of drug use in non-crash involved drivers because the refusal rate for drug testing in roadside controls typically exceeds the proportion of drivers who test positive for drugs, severely limiting data interpretation. Culpability analysis can overcome this problem when there are legal or medical protocols in place to determine drug use of crash involved drivers. The study of cellphone use while driving is another situation where obtaining good data in comparable non-crash involved drivers (or during a comparable ‘control driving period’ if using the case-crossover methodology) is a particular challenge. Our study employed culpability analysis to study the risk of crashes associated with cellphones for this very reason. Our findings were consistent with those derived using alternative study designs and support the growing understanding of the risks of driving while using cellphones. The term ‘culpability’ when used in the assessmentof crashes is drawn from its historical application as a means for determining legal fault; however, modern applications of culpability analysis have moved beyond this perspective to assess responsibility based upon a comprehensive set of indicators observed from the crash. Modern culpability studies, as well as considering whether actions of the index driver contributed to the crash, also look for other contributory factors such as road type, driving conditions, vehicle condition, contribution from other parties, crash type and difficulty of the task being performed. When actions of the index driver did not contribute to the crash and other factors did contribute, then the index driver is deemed ‘non-culpable’. It is the non-culpable drivers that are of greatest interest in a culpability analysis. As summarized by Wahlberg, culpability analysis is based on the assumption that drivers found non-culpable after a car crash represent a random sample of the general driving population that was ‘selected’ to crash by circumstances beyond their control and therefore have the same risk factor profile of other drivers on the road at the same time [odds ratio (OR)1⁄4 1.00]. If this ‘randomness assumption’ is met, then the risk estimate derived from a culpability analysis would be expected to be similar to that from a standard case-control study. To explain the assertion that culpability studies cannot help us understand crash risk due to cellphone use, Sanghavi presents a hypothetical scenario whereby cellphones prevent crashes and have a stronger protective effect during bad driving conditions. The author claims that this scenario could account for our findings and explains this by speaking of ‘culpable conditions’. We would first like to point out that Sanghavi’s hypothetical scenario where cellphones prevent crashes is inconsistent with extensive experimental research that demonstrates that Published by Oxford University Press on behalf of the International Epidemiological Association