The phrase "the vehicle ahead is travelling at km" is incomplete and seems to be missing the actual speed value. In general, when describing a vehicle's speed, it is usually stated as "the vehicle ahead is travelling at X km/h" where X is the speed in kilometers per hour. Kilometers per hour (km/h) is a common unit used to measure vehicle speeds, representing the number of kilometers the vehicle travels in one hour. Kilometers per hour is widely used because it's practical for everyday travel distances and times, making it easier for people to relate to speeds like 60 km/h (which means traveling 60 kilometers in one hour) rather than smaller or less intuitive units like meters per second.

If the intention was to report the speed of the vehicle ahead, the completed phrase would be something like "The vehicle ahead is travelling at 80 km/h," specifying the speed to indicate how fast it is moving.