Statistically, it seems like the police would very rarely, if ever, catch a person traveling with a lot of cash. First, a person would need to be pulled over (granted, isn't that rare), then the police would need to think to search the vehicle (they certainly aren't searching every out-of-town vehicle that is pulled over).
Yet, somehow they are managing to collect billions each year from civil asset forfeiture.
I suspect, without any proof what-so-ever, that police get records of large cash deposits/withdraws from banks and that information is available to them during traffic stops. These kind of cases happen too frequently to be chance, IMHO. There must be some kind of a priori knowledge available to the officers. Especially when they are nicking people going to pick up used cars from out-of-state or something.