The Invisible Drag of Traffic Tailgating

When a traffic light turns green, the line of cars does not surge forward as a single, coordinated unit. Instead, it moves in a ripple. The cars at the very front must react, shift into gear, and accelerate, creating a cascading delay that means the intersection's maximum discharge rate is only reached ten to fourteen seconds into the green light.

Drivers often try to squeeze every last inch of efficiency out of these moments by packing tightly together while stopped, eliminating the gaps between bumpers. Yet this instinct backfires. Packing tightly creates a domino effect of instability. When traffic is congested, minor perturbations in speed or spacing are amplified by each following driver down the line, turning a smooth start into a series of stop-and-go jerks.

This car-following behavior introduces a hidden bottleneck. Because drivers adjust their spacing based on speed to maintain a minimum time headway, flow becomes constrained by these longer headway times at the exact moments traffic is trying to recover. The resulting queue-discharge rate drops noticeably below the maximum capacity of the roadway.

In our rush to close every gap and push through a bottleneck, we inadvertently create the very friction that slows us down. The fastest way through a red light is often to leave a little room to breathe.