I hope you're spraying the handles and waiting a full 5 minutes for the disinfectant to evaporate. Otherwise it's a waste of time.
It's not absolute and that makes about as much sense as "the virus comes out after 10pm" jibe does. Its effect will be best if you do that. It's likely that doing something lesser will still have some effect.
Correct, it'll be an inverse exponent.
Washing your hands the way you always have is almost as effective as washing them very thoroughly whilst singing "Happy Birthday" twice, for similar reasons.
In the food industry, D-values are used to ensure food is safe to eat. For example the major risk with canned food is Clostoridium Botulinum. At 121C this has a D-value of 12 seconds, which means that there is a 1 log, or 90% reduction in the population of that bacteria in 12 seconds at that temperature. As you can never absolutely ensure that every last bacteria has been killed, the food industry accepted standard is "12 D". So your tin of beans goes through a retort which ensures the centre of the food reaches 121C for at least 144 seconds.
If you assume every can is contaminated prior to treatment, this means that 1 in a trillion will be contaminated after treatment.
If you treated them for 6D, one in a million would be contaminated.
It's the same with the 5 minutes or whatever the value is for sanitising the handle of a trolley. If 5 minutes is 12D, then 25 seconds of contact time is going to reduce any pathogen load by 90%. That's good enough for most people as you aren't likely to injest the trolley handle. And of course it would continue to work anyway as yo proceed round the store.
For the record, I view sanitising the trolley or basket handles as utterly pointless, given everything in the shop will have been touched by many people anyway. It's another of the facile measures I refuse to partake in.