言わんこっちゃない is a highly idiomatic contraction, standing for 言わないことじゃない ('it isn't the case that I didn't say so' = 'I told you so / what did I tell you'). The triple contraction (わない → わん, ことじゃ → こっちゃ) is characteristic of casual spoken Japanese, and the whole phrase functions as a fixed 'I warned you' exclamation. The preceding dialogue is a perfect setup: warning → accident → I-told-you-so. The と after 走る is the natural-consequence conditional.