The motorway was originally built with only six lanes (three each way), despite research which suggested eight lanes were necessary. Now a fourth lane exists over most of the motorway, and more in special sections (see diagram). Plans were scaled back in 2009 in response to rising costs, but work is still going on.
Public enquiries continued through the 1970s and 1980s. Each section had its own enquiry, where local citizens and interest groups could question the plans. In all, "39 public inquiries before independent Inspectors were held, taking over 700 sitting days – with many more working days (and nights) preparing evidence". Objectors could put forward alternatives, and each of these had to be answered with presentation of evidence. Evidence included predictions of traffic and events which would affect growth of traffic. Enquiries and evidence were open to public scrutiny and debate.
Some enquiries lasted a day, and one one lasted 97 sitting days over 13 months. Proposals had to go before Parliament, and a special Act was needed to authorise the crossing of the northern tip of Epping Forest by the route in tunnel.
As predicted by research, soon after the motorway opened in 1986 traffic levels exceeded maximum designed capacity. In 1990 the Secretary of State for Transport announced plans to widen the whole of the M25 to four lanes. By 1993 the motorway that was designed for a maximum of 88,000 vehicles per day was carrying 200,000. 15% of UK motorway traffic volume was on the M25 and there were plans to add 6 lanes to the section from Junction 12 to 15 as well widening the rest of the motorway to 4 lanes.
In 1995 a contract was awarded to widen the section between junctions 8 and 10 from six to eight lanes for a cost of £93.4 million and a 'Motorway Incident Detection and Automatic Signalling' (MIDAS) system was introduced to the M25 from junction 10 to junction 15 at a cost of £13.5m in 1995 and then extended to junction 16 at a cost of £11.7m in 2002. This consists of a distributed network of traffic and weather sensors, speed cameras and variable-speed signs that control traffic speeds with little human supervision. The system has improved traffic flow slightly, and reduced the amount of start-stop driving. The use of variable speed limits makes this a "controlled motorway", and if hard shoulders are used at peak times it becomes a "managed motorway".
In 1995 there was a proposal to widen the section close to Heathrow Airport to 14 lanes. This attracted fierce opposition from road protesters opposing the Newbury Bypass and other schemes, and it was cancelled shortly afterwards. However, in 1997 the Department of Transport announced new proposals to widen the section from junction 12 (M3) and junction 15 (M4) to 12 lanes. At the Terminal Five public inquiry a Highways Agency official said that the widening was needed to accommodate traffic to the proposed new terminal, however the transport minister said that no such evidence had been given. Environmental groups objected to the decision to go ahead without holding a public inquiry. A decision to go-ahead was given for a 10-lane scheme in 1998, and the £148 million 'M25 Jct 12 to 15 Widening' contract was awarded to Balfour Beatty in 2003. The scheme was completed in 2005 as dual-five lane between junctions 12 to 14 and dual six lanes from 14 to 15.
The road cost £1 billion (in 1980s money) and took 11 years to build. It used two million tons of concrete and 3½ million tons of asphalt.
Traffic bottleneck at the Dartford Crossing in the east is the greatest problem on the motorway at present, even though the crossing is technically not part of the M25. At present the crossing is by bridge from the north, and by tunnel from the south. There are usually long tailbacks in both directions.
On its 25th birthday the BBC listed some of its effect on life near the capital:
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Harold Urey
Harold Clayton Urey (April 29, 1893 – January 5, 1981) was an American physical chemist. His pioneering work on isotopes earned him the Nobel Prize in Chemistry in 1934. He played a significant role in the development of the atom bomb, and he had ideas on the development of organic life from non-living matter.
Urey earned a degree in zoology from the University of Montana and a Ph.D. in chemistry, studying thermodynamics under Gilbert N. Lewis at the University of California, Berkeley.
At Berkeley, Urey was influenced by the work of physicist Raymond T. Birge and soon joined Niels Bohr in Copenhagen to work on atomic structure at the Institute for Theoretical Physics.
On his return to the U.S. and between 1924 and 1928, he taught at The Johns Hopkins University as 'Associate in Chemistry', and then at Columbia where he assembled a team of associates. He wrote (with Arthur Ruark), "Atoms, quanta and molecules", one of the first English texts on quantum mechanics and its applications to atomic and molecular systems.
He discovered deuterium, isolating it by repeatedly distilling a sample of liquid hydrogen. In 1931, he and his associates went on to demonstrate the existence of heavy water. Urey was awarded the Nobel Prize in Chemistry in 1934 for this work.
During World War II, Urey's team at Columbia worked on a number of research programs that contributed towards the Manhattan Project to develop an atomic bomb for the United States. Most importantly, they developed the gaseous diffusion method to separate uranium-235 from uranium-238. In autumn 1941, Urey, with G.B. Pegram, led a diplomatic mission to England to establish co-operation on the development of the atomic bomb.
Isaac Asimov, a student at Columbia at this time, remembers Urey lamenting, perhaps too strongly, how pained he was that he could do nothing to help the war effort. Asimov pointed out innocently that perhaps the enriched uranium kept at Columbia may have had something to do with the war effort. Urey reddened and changed the subject. Urey had worried about the German atomic bomb while working on uranium separation at Columbia during the war, and now referred to "the most dangerous situation that humanity has ever faced in all history".
After the war, Urey became professor of chemistry at the Institute for Nuclear Studies, then Ryerson professor of chemistry at the University of Chicago before moving to the University of California, San Diego. A UCSD building was named in his honor in the early 1960s, during a time when nearly all buildings other than student dormitories had only generic names. The name of the building is actually the "Frieda and Harold Urey Hall". Urey would have rejected the honor, he hated the architecture of the building, but since it also honored his wife he accepted.