I first thought it to be local skater slang for getting together for a “smoke,” but I have certainly heard it used by others in the city surrounding areas, and elsewhere. Did the term originate elswhere? Is it used elswhere?
About a hundred years ago ;), when I worked in construction, it meant having a beer or two in the nearest liquor store parking lot.
Toward the end of the day someone would say “Safety meeting at the 7-11.”
We were so cool. 
Peace,
mangeorge
Although I hadn’t heard the ‘Safety meeting’ part, This link has many references to 420 and smoking weed. It’s still a fairly underground expression around here, and I don’t really grok the relationship between the number and the act. Maybe there’s more in one of the Google links - I haven’t checked that deep.
-mdf
I think this has come up on the boards before.
According to Steven Hager, editor of High Times, the term 420 originated at San Rafael High School, in 1971, among a group of about a dozen pot-smoking wiseacres who called themselves the Waldos, who are now pushing 50. The term was shorthand for the time of day the group would meet, at the campus statue of Louis Pasteur, to smoke pot. Intent on developing their own discreet language, they made 420 code for a time to get high, and its use spread among members of an entire generation. While our teens feel that they know something we don’t, you can let them in on the fact that it was your generation that came up with the numbers.
A quote from one of the Waldos in the High Times article states, “We did discover we could talk about getting high in front of our parents without them knowing by using the phrase 420.” Fortunately, your teenagers will not have that same option.
I think Road Rash is asking about the ‘safety meeting’ part, not the 420 part. Unless I misunderstand the comma. 
Is that correct, RR?
Peace,
mangeorge
I was referring to the expression “safety meeting.” “420” is incredibly common. They even have shops called “420” that sell accesories now.