closing speed

noun

noun

The speed of a moving object relative to another object which it is approaching.

If two vehicles, both travelling at 70 mph (miles per hour), collide head-on, their closing speed is 140 mph.
This secondary collision, head-on with a closing speed of 142mph, caused the DVT to veer off to the left. Many of the coaches behind it overturned and careered into an adjacent field.2021 February 24, Greg Morse, “Great Heck: a tragic chain of events”, in RAIL, number 925, page 39:

Entry derived from the Wiktionary, under licence CC BY-SA 4.0 — list of authors.