Firebreaks are a long-standing fire management technique. The problem is that with an intense fire, it's possible for the flame to leap across the firebreak, even very wide ones when there are windy conditions (which an intense fire can exacerbate).

https://en.wikipedia.org/wiki/Firebreak

A greater risk than flames crossing firebreaks is firebrands or cinders - flaming bits of bark, small branch, clumps of pine needles - being carried aloft by heat and falling past the firebreak into combustible dry grass or other fuel, or onto a wooden deck next to wooden siding of a house, or in a rain gutter full of pine needles and twigs next to a composition shingle roof edge.

Yeah, sloppy language on my part. I should have said "fire" rather than "flame", the former encompassing firebrands/cinders/etc.

I mean Santa Rosa got hit by fire after flames jumped the multi-lane I-5 freeway a few years ago, so I don’t think small gaps / roadways accomplish anything in some of these fires.

During the socal fires a few years back, the Santa Ana winds caused it to jump an 8 lane freeway. You're going to need one hell of a firebreak.

A fire jumped the Columbia River (a large river) a couple of years ago

https://www.opb.org/news/series/wildfires/oregon-columbia-ri...