Some of the recent protests over Furguson have attempted to block traffic (LA and Boston) to cause delays. Here's an example (note the barricades on the left). Of course, this method isn't much of a danger.

A more troublesome method?

Disruption that uses a very old technology: the caltrop (see below).

Caltrops were originally designed to damage the hooves of horses (or impale the foot of a soldier). They work equally well against tires.

A handful of these tossed onto a highway at periodic intervals and in different locations can achieve very high levels of disruption.

Not only that, they are actually very easy to make. Just clip a section from a chain fence. Clip the ends to a point and bend them into shape.

PS: Years ago, I pointed to a study by the Federal Reserve that showed that disruption like this can act as a "tax" on a urban target that can cause a severe economic contraction. The trick is keeping it going long enough to happen.

PPS: Here a vehicle (with a false bottom) that was used to disrupt the main N-S highway in China last year. Was it the cause of the 120 mile/week long traffic jam?