I don't use the M42 but was curious and found this description of the system:
http://www.highways.gov.uk/roads/projects/.../consult/01.htm
Being a sad git, I went on to find out about HADECS and found this document regarding the M25 scheme:
http://www.highways.gov.uk/news/press_rele...mary_report.pdf
Until I read the above, I thought that the effect of the ATM (active traffic management) on the M25 was very positive, except when speeds were incorrectly set. I have experienced journeys where the traffic flowed freely in the 40 MPH zone but was stop-start when I left it. However, it seems that the real benefits are quite small and, in some cases, there is no benefit. Even if you don't wade through the whole document, take a look at the summary, Table 2A. Here's one extract:
"There has been no increase in the peak 1-hour throughput. However, the total throughputs during the 5-hour peak periods on the M25 Junctions 15-16 have increased by approximately 1.5%. There will be a benefit if this greater demand can be accommodated without a drop in performance."
There are safety benefits in the reduced severity of accidents but most other effects are much smaller than I imagined. Maybe I was a victim of spin? The report does support Rillo's comment
However, it does also show that if the speeds are set correctly it ATM can work.