Buchanan finished in 38.433, 0.210 seconds ahead of New Zealand's Sarah Walker (38.644), with Mariana Pajon of Colombia third in 38.787.
French rider Laetitia le Corguille was fourth and Great Britain's three-time world champion Shanaze Reade fifth.
World champion Magalie Pottier of France ranked seventh after the event, which determined seedings for Friday's semi-finals.
The riders hurtled down the start ramp one at a time in a bid to claim a strong ranking for the latter stages of the competition, when eight riders will vie for position on an undulating course, full of jumps and tight turns.
Reade was third of the 16-strong field to take to the route, which was modified after she won the test event last summer, and was the provisional leader for a time.
Fifth rider Walker bettered her mark before penultimate rider Buchanan went quicker still.
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