Guardrail washer?

[fredtoast]

On the way to work today I saw a water truck driving along the side of the road with a huge nozzle spraying the guardrail. Looked like it was designed specifically to wash off the guardrail. I though it might be some sort of weed killer they were trying to spray on the right-of-way, but it looked like high volume of water instead of a fine spray like you usually see with chemical sprayers. Also it would have been very poorly designed as a weed sprayer because it was spraying most of it directly at guardrail level.

Never seen anything like that before, and I don't know why it would be necessary to wash guardrails.

How about you guys?