In the first stage of a pounds 16m scheme to help drivers dodge delays, parts of the M1, A1, A1/M and the M25 will carry the signs which can convey messages of up to 54 characters.
The system will then be launched on the M11 and in the West and East Midlands by September. It is operated from police control centres in Welwyn Garden City and Perry Barr in the West Midlands.
The project builds on the experience of similar schemes in Kent and around the Birmingham motorway box. Typically, messages will advise drivers of bottlenecks and suggest alternative routes to bypass delays and trouble spots.
For example, a motorist travelling from the Dartford tunnel on the M25 to the West Midlands or the North has four alternative routes in the event of a hold up or bad weather: M25, M11 to A14; M25, A1, A14; M25, M1; or M25, M40.
The project is part of the Government's move away from building roads and instead investing in ways of improving the existing road network. This was one of the themes of the Green Paper on transport published earlier this week.
Lawrie Haynes, chief executive of the Highways Agency, who switched on the system yesterday, said: "Delays and congestion are extremely costly to business as well as being very frustrating to all road users. But it is often the case that, while one stretch of road is congested, there is spare capacity on another parallel route.
"We need to make more effective use of the existing road network. This means using the latest techniques and technology to make the most of past investment and to ensure that environmental effects are kept at an acceptable level."