Riding on roads?
I want to go to the work (in São Paulo, Brazil), by bicycle. But I live in a neighbor city, and I need to get a high traffic road.
What is your experience with this, and could it be safe?
I live in Brazil too (Porto Alegre), and I would adivise you to use only roads with wide shoulders ("acostamento") or avenues. Unfortunately, as the vehicles' size (trucks, buses) get bigger than the lanes, biker safety tends to zero.
The most important device I installed in my life is a big helmet-mounted rear-mirror. It took me two weeks to automate it's use, and another six months to cope with the anxiety of looking all those vehicles coming "straight" after me, something I didn't care so much while I was not able to actually see it. Now I've "mastered" its use, I almost nothing look behind turning the head. But sometimes I look more to the mirror than to the way ahead... VERY useful and HIGHLY recommended.
Also, the faster you ride, the safer you are, specially on saturated/near-congested traffic, because you can choose your lane positioning without tempting the rear drivers to overtake you. So, it's worth to invest on a relatively light bike, with relatively narrow/hi-pressure/slick tires, and a pair of strong legs :o)
But, like Guilherme Santos said, things are getting better, slowly. At the same time, unfortunately, dead cyclist news are not uncommon. But, what choice one has but to ride?
As Chris says, a lot depends on the road, the traffic, and your riding style. If you have a road with a generous shoulder then you can usually ride on that safely in even the heaviest traffic.
But if you must ride in the driving lanes a big part of safety is knowing how to "claim your lane" and having the courage to do so. This means riding well towards the center of the lane rather than trying to stay over towards the edge.
But we here in the US have no way to judge the nature of the traffic (or the drivers) in Sao Paulo. If drivers are too aggressive/macho then it becomes unsafe to cycle in traffic.
Riding here (Vancouver, Canada) in the city is quite different from what you're talking about (i.e. no highways and "high-traffic roads" would be the main arterial streets), but I still thought I'd chime in...
Personally, I avoid heavy traffic whenever possible. I don't consider the amount of time I'd would save by riding on a major street to be worth the risk. I find it much more pleasant to ride on a quiet side-street than a busy road. There's probably more risk of a car blasting through an intersection on an side street, but on the other hand I can hear them coming.
It can be safe and/or it can be dangerous: depending on the road, the time of day, and how you ride. I have been hit once (from behind, not my fault).