They would be most successful where population and sign density is high, which would be the cities. During such an attack, public transport could move around pe

They would be most successful where population and sign density is high, which would be the cities. During such an attack, public transport could move around people while the authorities would take care of the attackers. The problem for them is that where they would cause the most harm, rural areas (since they are in most dire need of supplies a few towns away and don't have any good alternatives to cars), is where such an attack would be the hardest to implement. Canada, Australia, Iceland and Alaska has many roads where vital street signs can be tens of miles apart from each other, as well any actual people who might be effected by this. Also, demographic movements is working to their disadvantage; more and more people everyday are moving to large cities.