https://phys.org/news/2018-06-fuego-volcanothe-deadly-pyroclastic-dozens.html

Footage taken from a road bridge over a dry valley from on June 3 at Fuego shows what appears to be a soft and billowing ash cloud gently flowing down the volcano. It looks innocuous. Spectators and officials watch mesmerised, but then the cloud moves into the valley and heads directly towards the bridge. Unease spreads and soon alarms sound before people rush away just in time. Footage shows the ash cloud quickly pass under and then over the bridge. These spectators escaped death by seconds, as this benign-looking ash cloud is the notorious killer that is a pyroclastic flow.

Pyroclastic flows (also known as pyroclastic density currents) contain a hellish combination of hot rock fragments (pyroclasts), superheated air, and volcanic gases. You can expect temperatures of 100-600°C and they can travel fast – very fast on steep slopes. Speeds generally range between 70-200mph, but they have been recorded reaching 450mph. As they are heavier (denser) than air, they tend to be funnelled into valleys. But their higher density also gives them momentum, so they can travel up the sides of valleys, and even over mountains. The worst place to be when a pyroclastic flow is on the move is in a valley, which is why the spectators at the road bridge were lucky to escape.