Surely it’s simpler to just hang a sign on chains at just the height in question about 50m in front of the bridge so that anyone driving a vehicle that is too high will be rudely awakened.
Edit here is an example of the kind of thing I’m talking about: http://www.heraldsun.com.au/news/victoria/montague-street-bridge-warning-gantry-installed-after-it-was-hit-more-than-100-times/news-story/0645bf542c8c2805e83048b628e84b0c
