Zhège xuéxiào de xuésheng rénshù bǐ wǒmen xuéxiào duō yí bèi. (= Zhège xuéxiào de xuésheng rénshù shì wǒmen xuéxiào de liǎng bèi.)
The student number of this school is twice as big as that of our school. (—> The student population of this school is twice of ours.)
多一倍 is translated, first in Chinese as 两倍 so it's easy to see how confusing this phrasing is! But....
② [indicating difference in quantity]
The output of rice is three times that of wheat in this area
两倍 has become three times here(!).
but(!) - still from Oxford:
Your salary is twice that of mine.
Basically you've really got to look at the context to figure out the meaning!
Your sentence above
开车最少比骑自行车快一倍。 the 快 here has the same effect as 多 so it's really saying it's "one time as fast" which is really "twice as fast"...
Hope that makes sense