One of the most important factors in UI design is consistency. Consistency within a single product/object and consistency between various product lines or objects built by the same company.
All of us have been in a two-lane turn onto an on-ramp, in which after you get onto the ramp, one of the two lanes ends. The problem is - you never know which one of those lanes is actually going to end, and this results in a little bit of confusion when everyone finally realizes which lane does in fact come to an end on the ramp.
Here is an example of two lanes of traffic turning left onto the on-ramp from South Saunders Street in Raleigh, NC, merging onto the inner beltline of I-440. If you look closely you can see that the right lane ends.
And here is an example of two lanes of traffic turning left onto the on-ramp from 15-501 in Chapel Hill, NC, merging onto I-40 West. Here you can see that it is the left lane that ends:
All multi-lane on-ramps (at least within the same state) that cut down into a single lane should do so in the same fashion. Either always end the lanes on the left or always end the lanes on the right. Alternatively, always end the lane farthest from the road you're merging onto.