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The evidence of/from all parties is that John's vehicle struck Mary's [vehicle] as/when he turned left onto Highway 1.
If you say "John struck Mary's vehicle" it sounds like he hit it with his hand/fist, not his car.
And I took a guess that you meant his vehicle was turning left in order to gain access to Highway 1 (onto), not that he turned left from Highway 1 to take a different road. If that was the case, then he turned left "off" Highway 1.