Previously, the primary method of vehicle detection was done by cutting the pavement a few inches and installing a wire "loop" just below the surface.
These in-ground loops have both near-term and long-term costs. The labor for a work crew to saw-cut pavement while shutting down that lane of traffic, and the cost of material are immediate costs. The saw cutting of the pavement weakens its strength, resulting in shorter service life and more maintenance costs for pavement repair. When in-ground loops fail, the entire loop must be recut into the pavement again. Video cameras give us low cost options to change the detection boxes do to construction or lane shifts, while pavement loops do not.
Video detection cameras, like most electronics, have seen their costs steadily drop since the technology was first introduced. Installation is done above the surface of the road, and usually away from the flow of traffic. The pavement remains undisturbed and is capable of lasting longer. When cameras or processors fail, they are simply and quickly replaced without a great disturbance in traffic flow.