There are specific horn signals (one prolonged, or seven-short-one-prolonged, are what I'd guess would be appropriate here), and COLREGS do explicitly say that

There are specific horn signals (one prolonged, or seven-short-one-prolonged, are what I'd guess would be appropriate here), and COLREGS do explicitly say that you can use whatever you need to get attention in an emergency as long as it's not confusable with some official signal. But as other commenters have noted it wouldn't have been specific enough to get the workers etc to clear the bridge before the impact.