Does anyone know how this referendum came about? Was it enacted by the 
local government or was it a "citizens' initiative"?

18.9.2006
Stockholm voters approve contentious traffic congestion toll

AP - Stockholm voters approved a new road toll system designed to reduce 
traffic, noise and pollution, officials said Monday. Near-complete 
results for the Sunday referendum showed that 51.7% of Stockholm voters 
approved the traffic toll, while 45.6% voted against it. The referendum 
in the Swedish capital took place in conjunction with national elections 
for parliament that saw a center-right alliance voted in and ousted 
Social Democrats after 12 years in power.
The congestion fee was contested when city officials introduced it in a 
seven-month trial that ran between January and July. But public opinion 
swung in favor of the charges after studies showed that weekday traffic 
on average dropped 20 percent during the trial, while pollution 
decreased 9-14 percent. Drivers had to pay a fee when entering and 
leaving the city during rush-hour. Similar systems are already in place 
in other cities, including London, Oslo and Singapore (…).

Regards

Wallace-Macpherson
I&R ~ GB Citizens' Initiative and Referendum
a campaign for direct democracy in Britain
http://www.iniref.org/