I find sunglasses more flexible than dark shields. For a sunny trackday, I'll put on a dark shield as it provides more field of view and I know there is no chance of being caught out after dark. But on the street, a clear shield + sunglasses allows more airflow when needed, and the ability to have no tint at all if you do end up riding longer than expected. It's much easier than finding a place to carry an extra clear visor on the bike. In terms of safety, I wouldn't wear prescription glass lenses, but any decent pair of sunglasses now is impact-resistant / shatter-resistant.
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