The recent vehicle-based terror attacks in London and Stockholm have been noted as much for their devastation and chaos as for its low-tech approach to terrorism. At the core, these attacks are predicated upon a very simple premise: drive a regular civilian passenger vehicle through crowds of people in a very public, high-profile, and undefended area -- colliding with as many individuals as possible -- with some form of knife-based or small-arms attack afterward, if desired. As devastating and chaotic as these are, this is not a new form of attack.
The problem, however, is that this form of attack is so simple and effective. These attacks can be executed with very little planning, no training, no funding, and no preparation, which makes them quick, easy, and deadly. Given the simple and self-contained nature of these attacks, there is very little law enforcement, the intelligence community, or even the military can do: when a future terror actor decides to undertake this effort, they simply don't raise any red flags that would trigger various forms of government surveillance or contact.
Kyle R. Brady