It is not down to Euro constitution, sorry, as they discussed on C4 News yesterday the failure of the constitution doesn't actually change anything - all the existing treaties (Maastricht, Rome etc) are still in place.
It would appear that it might be a part of the detailed work following on from this 2001 report about Energy and Transport - Andris Piebalgs is in charge of the energy part and works closely with transport...
http://europa.eu.int/comm/energy_transport/en/lb_en.html
Here in writing is the Euro grand plan for transport - I haven't read it all, but I have skimmed a few sections and it covers everything from modal shift (forcing you to use public transport), to the banning of Lorries at week-ends, extending speed restrictions to buses (bus >= 8 people), common road signs, common tax policies, road usage charging, aircraft fuel taxation etc.
Some of it appears to be very sensible, and then it starts going on about costs with some stacked bar graphs (p106/107) where somehow Car use is dramatically more costly that rail or bus although it does admit that bus use causes more polution that car! Where I really start to question some of the connections is blind statements about CO2 being the
primary cause of global warming (big fan of Kyoto I think), but they don't blink an eye about suggesting Hydrogen cars which only output water vapour; is this water vapour that is the real no1 greenhouse gas???
Will it happen? well the timetable states 2005 is the big decision date, and that European Parlement and Council both need to agree, and that the Transport ministers current policy of seeking concensus must be broken allowing the decision to be made by "qualified majority".
Having looked at this thing and thought about the timing, it is most likely that this speed limit statement is actually a "softener" before the big debates begin so that when they don't do it we are all happy to have all of the other dodgy measures slipped through.