Looming and Closing Speed

What looming is

Looming is the growth of an object's image in a driver's view as the distance to it shrinks. It is the main visual cue drivers use to judge closing speed: how quickly they are approaching a vehicle ahead, or how quickly another vehicle is approaching them.

At long range that image grows very slowly, even when the difference in speed is large. A driver can see a vehicle ahead, and can tell that the distance to it is shrinking, well before being able to judge how fast. From a distance, a stopped or slow-moving vehicle can look much like one moving with traffic. Research describes this as a limit of human perception, one that applies to attentive drivers as well.

How it bears on driving

Looming questions arise most often in high-speed rear-end collisions with a vehicle that is stopped, disabled, or moving well below the speed of traffic, such as one that has just entered the roadway. They also arise when a driver has to judge an oncoming vehicle before turning or pulling out across its path, particularly when that vehicle is narrow, such as a motorcycle.

Other cues can tell a driver that a vehicle ahead is slow or stopped before looming does: brake lamps and hazard flashers, the behavior of surrounding traffic, warning devices, and the setting itself. At night there are fewer such cues, and a driver may have little more than a pair of tail lamps to judge by. Whether a driver responded within a reasonable time therefore depends on what information was available and when. Depending on the facts, the analysis can point in either direction.

Questions a looming and closing speed expert witness can address

• When could a following driver tell that the vehicle ahead was stopped or moving slowly, as distinct from seeing that a vehicle was there?
• What cues, apart from looming, were available to signal the difference in speed?
• Once the closing speed could be perceived, did the driver respond within the time the research would anticipate?

This article is general information, not legal advice or an expert opinion on any matter. See the disclaimer.