Here's the scenario without any mines. The convoy successfully drives north along the Coast Road and turns right at the Crossroads to reach Levie.
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The exact same scenario, except the Crossroads have been mined. Instead of driving north, the Convoy does a U turn and travels South, crosses the pass, and goes to Levie via the Green Valley road.
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On further investigation, it turns out that even if the mines are placed AFTER the convoy has started travelling up the NORTH coast road (but still miles out of visual range), it will detect them as soon as they are laid and do an immediate U-turn to take the alternative route SOUTH over the mountain range.