Transmission line (TL) backflashover (BF) performance has been traditionally ascertained using a single number, the so-called backflashover rate, which is measured in number of BF events per 100 km-years. This paper aims at presenting a novel indicator for assessing performance of high-voltage TL tower’s ability for tolerating direct lightning strikes without provoking BF events. This indicator is also defined as a single number, which can be computed for any TL tower (by means of EMTP simulations) and measures, in a novel way, its tolerance against BF events. It is given in terms of the risk of the BF occurrence, which means it is statistical in nature and depends on the total sum of conditions governing the BF events. The BF risk, as an indicator, is obtained from the probability density function of the shield wire(s) incident lightning currents and the cumulative distribution function of the BF currents statistical distribution. Hence, it merges complete probabilities of obtaining lightning currents striking a tower with probabilities of those currents provoking BF events on that tower. This novel risk-based indicator can be related to the price of that risk and the associated investment costs, enabling the cost-effective optimisation solutions to the problems of TL arrester applications and station insulation coordination design. This seems appropriate, considering the fact that the investment in surge arresters and related protective measures is commonly perceived as buying insurance.