So, we are 20 minutes into the incident, the queues are probably already four miles long on the southbound side, and there is a similar queue on the northbound carriageway caused by rubber-neckers.
The Network Control Centre will call the Incident Support Unit closest to the scene. These vehicles have tracking devices on board, so a controller at the Network Control Centre can see where all Incident Support Units are at at any one time. Depending on the contract details, the Incident Support Units could be anywhere on the network working, they could be stationed close to accident clusters at rush-hours (obtained from RTA data), or strategically placed on the network for a rapid response at any time of the day. (By way of explanation, depending on the route and time of day/night, the HA contract will give a maximum response time of between 1.5 hours and 20 minutes, the latter being the M25 where crews are strategically positioned waiting all day).
If the Police have already requested closures or diversions the Network Control Centre will call the Supervisor/agent for Term Maintenance Contractor and he will need to sort this out. He may be lucky; a crew may be in the compound loading up with materials, in which case it's a case of "forget that, we've got an incident and the road needs closing". As part of the contract, sign trailers are stored in the compounds fully loaded to close a motorway or trunk road, so it's just a matter of hitching this up and commencing the signing coning from an agreed marker post location. The Supervisor/agent will also have to make arrangements to get somebody else to attend this crew's original work site to make that site safe and remove any traffic management they had in place.
However, things rarely run smoothly, so it is likely that the crews he needs to carry out the closures are out on the network carrying out other day-to-day maintenance tasks. He will know where the crews are and will call that crew to attend the incident. The nearest crew may not be the best suited to attend, they may have to go past the scene to get to the compound, the site may not be in a position to be closed down immediately; many factors come into play. It may be easier to get a operative that is road sweeping or gully emptying to come in, hitch up the trailer, then pick up this crew en-route to the incident. Again, these existing work sites will be need to be made safe and traffic management removed, but the road closure must take precedence at this time.
Once the Network Control Centre have called the Incident Support Unit and agent, they next call the Client Supervisor to attend. They will give him as much information as possible about the incident itself, such as what they have been asked to provide, what they have done so far, and an idea about ETA's etc.
As the Network Control Centre's primary function is to take the call from the police and provide the initial response, it usually means that the Client Supervisor then has to ring the Police to get more detailed information on the incident so that he can instruct further attendance of crews, arrange any additional plant such as mechanical sweepers, gully emptiers or specialised equipment. He will find out if "trigger signs" for the diversion have been activated, and if not, he will do it, or arrange for it to be done by others en-route. He will then of course leave to attend the scene. Depending on where he is, he could be the first HA representative to arrive at the scene, or it could be the Incident Support Unit (there have been instances where either or both have arrived before the emergency services - good old cutbacks and paperwork!).