ðeɪ ɑːɹ dʒˈɛnɚɹəli jˈuːzd ɐlˈɔŋ ˈɛɹiəz ʌv ɹᵻstɹˈɪktᵻd klˈɪɹəns, sˈʌtʃ ɐz ɐ bɹˈɪdʒ, tɹˈɛsəl, tˈʌnəl, ɔːɹ lˈɛvəl kɹˈɔsɪŋ.
They are generally used along areas of restricted clearance, such as a bridge, trestle, tunnel, or level crossing.