Something similar happened in my school back in April 1997.
The IRA placed several real semtex bombs and a few fakes onto the M1, M5 and M6 motorways. One of the bombs was near Junction 10a of the M6, which is only a few hundred yards away. We weren't allowed to go outside or stand near windows. The police evacuated us at lunch time, after which the bomb squad carried out a controlled explosion.
It was quite weird as J10 is one of the busiest section of the motorway in the country, and is naturally quite noisy. On that day there was just an eerie silence.