Attention is now turning to the way diesels are tested for NOX emissions after Mercedes, Honda, Mazda, and Mitsubishi cars all failed under real world conditions.
http://arstechnica.com/cars/2015/10/95-of-european-diesels-tested-flunk-emissions-standards/
The European test protocol (
NEDC) isn't particularly realistic - and features quite gentle acceleration (e.g. 0-31 mph in 26 seconds for the urban cycle and 0-43 in 41 seconds for the extra-urban cycle).
It's not impossible that this protocol simply doesn't result in very high emissions due to the low stresses on the engine. I know that I don't drive like that except in bumper-to-bumper traffic, whereas the overall urban NEDC cycle is much more free flowing.
However, because the cycle is quite stereotyped, it could also be possible to tune the engine most aggressively for the operating envelope of the test cycle, which would also result in higher real world emissions.
The EU legislators have been working on trying to get a "real world" driving cycle introduced, which would be much more complex and feature a much wider pattern of acceleration/deceleration and cruise speed with the aim of more accurately modelling real world driving and making "tuning for the test" much more difficult.