I don't like the bridge example because the values were chosen (intentionally or not) conveniently for metric. Change it to every 4 feet or 1.3 metres and it's no longer convenient in either system. There are better examples that demonstrate the superiority of metric.

For example, pool cleaner says 1 unit per 10,000 gal or 40,000 L.

21' diameter, 3' tall. So ~1000 ft³. Multiply by 1728/231 for gallons.

7 m diameter, 1 m tall. So ~40 m³. Multiply by 1000 for litres.

If you're curious where 1728/231 comes from, there are 12³ (1728) in³ for a ft³. Then the gallon is defined as 231 in³