We had an M2 D228 on a positioning flight inbound to IoM the other day which lost power on one engine. There was a full turnout of ambulance and off airport fire appliances. And they closed the road running past the airport.
I cannot find out why such a high level of response was needed, especially when we are constantly reassured by the pilot community that losing one engine is a non-event. I'm also puzzled at the addition of non airport fire appliances to the response. Either the on airport resource is adequate or it is not. And if it is not then something must be amiss.
But the OP referred to the closing of the road alongside BHD and the same thing happened here in IoM. It seems that the emergency planners (who have nothing to do but think up scenarios and solutions, and based on real life events, they are usually wrong - Katrina is a fine example of this) have decided that for any incident the road closure is essential.
I suspect (although no one has confirmed this) that it flows from the Trident accident in Staines alongside the M4 many years ago when the rubbernecking public jammed the access for the emergency services.