I don't think this should normally be the focus of the game. For one, it's just not interesting to constantly hammer players with petty mishaps. If they are strangers in a new area they don't understand, and the area is particularly messy in some way, like garbage being dumped frequently (every few minutes) from upper story windows, then yeah, it might come into play. It's like the idea of rolling to successfully drink a glass of water; unless there's some reason why this matters, why are we doing this, exactly?
The other reason why it wouldn't normally be an important thing in my games is because I figure characters in a fantasy setting are familiar with the everyday habits of that setting. If people dump filth into the streets a couple times a day, you'd think that people would normally be aware of this. All you should be interested in are the exceptions.
And so, I settled on this method: as long as a character's Intelligence, Wisdom and Dexterity are all above 8, there's no need to worry about accidents in everyday situations. If, however, one of those three is 8 or less, roll 1d6: 5+ means an accident. Modify the target number based on the scores: +1 per score of 13+, -1 per score of 8 or less. So:
- Alice (Int 8, Wis 10, Dex 10) knows people dump filth in the streets in this part of town, but doesn't always know where to walk to best avoid it; half the time (4+ on 1d6,) she gets splattered with muck.
- Bob (Int 10, Wis 8, Dex 8) knows how to pick a safe path, he's just not wise enough to realize he *ought* to, and kind of clumsy; he gets splattered 2/3rds of the time.
- Carol (Int 10, Wis 13, Dex 8) is slow to react, but has average smarts and is wiser than normal, so she's only splattered 1/3rd of the time.
- David (Int13, Wis 8, Dex 13) is just a little foolish, but otherwise above average, so he gets splattered 1/6th of the time.
- Eva (Int 10, Wis 10, Dex 13) is average or better, so she normally doesn't test at all. If something happened to make her more vulnerable (temporarily blinded and pushed out into the streets on the day of the Festival of Mass Dumping,) she'd make the test, but at +1, the same as David.
A curse would also trigger checks where normally there wouldn't be any.