Driven to despair

Annex A: The Canadian Risk of Harm Formula

The risk of harm (RH) to other road users posed by the driver with heart disease is assumed to be directly proportional to the following:

a. time spent behind the wheel or distance driven in a given time period (TD)

b. type of vehicle driven (V)

c. risk of sudden cardiac incapacitation (SCI)

d. the probability that such an event will result in a fatal or injury-producing accident (Ac)

Expressing this statement as Formula 1: RH = TD x V x SCI x Ac

In Canada fewer than 2% of reported incidents of driver sudden death or loss of consciousness have resulted in injury or death to other road users or bystanders. In Formula 1, therefore, Ac = 0.02 for all drivers. 

There is evidence that loss of control of a heavy truck or passenger-carrying vehicle results in a more devastating accident than loss of control of a private car. In Canada vocational drivers are involved in only about 2% of all road accidents but in approximately 7.2% of all fatal accidents. In Formula 1, if V = 1 for a vocational driver, then V = 0.28 for a private driver.

There is no published standard or definition of what level of risk is considered acceptable in Canada even though this is crucial in the formulation of guidelines based on the probability of some event occurring in a defined time period. It was necessary, therefore, to develop such a standard. 

For several years, the guidelines of the Canadian Cardiovascular Society, the Canadian Medical Association, and the Canadian Council of Motor Transport Administrators have permitted the driver of a heavy truck to return to that occupation following an acute myocardial infarction provided that he or she is functional class 1 with a negative exercise stress test at 7 metabolic equivalents, has no disqualifying ventricular arrhythmias and is at least 3 months post infarct. 

On the basis of available data, however, such a person cannot be assigned a risk lower than 1% of cardiac death in the next year. The risk of sudden death would be lower than this but would be at least partially offset by the risk of other suddenly disabling events such as syncope or stroke. For such a person, SCI is estimated to be equal to 0.01 in Formula 1. It may be assumed that the average commercial driver spends 25% of his or her time behind the wheel (5). Thus, in Formula 1, TD = 0.25. As indicated above, V may be assigned a value of 1 for commercial drivers and Ac = 0.02 for all drivers. 

Substituting into Formula 1: RH = TD x V x SCI x Ac = 0.25 x 1 x 0.01 x 0.02 = 0.00005

Allowing such a driver on the road is associated with an annual risk of death or injury to others of approximately 1 in 20,000 (0.00005). This level of risk appears to be generally acceptable in Canada.

A similar standard may be applied to the driver of a private automobile. The average private driver spends approximately 4% of his or her time behind the wheel (TD = 0.04). As indicated above, for such a driver, V = 0.28 and Ac = 0.02. The acceptable yearly risk of sudden death or cardiac incapacitation for such a person would be calculated as follows:

RH = TD x V x SCI x Ac 0.00005 = 0.04 x 0.28 x SCI x 0.02 SCI = 0.223

Thus, the private automobile driver with a 22% risk of sustaining an SCI in the next year poses no greater threat to public safety than the heavy truck driver with a 1% risk.

Finally, for the commercial driver who drives a light vehicle, such as a taxicab or delivery truck, V = 0.28 and TD = 0.25, placing them at a risk between that of the private driver and the tractor-trailer driver.

Source: Final report of the Canadian Cardiovascular Society Consensus Conference 2003.