The best transition word or phrase to clarify the relationship between the ideas in these sentences is "for example."
So, the correct combined sentence would be:
"In some big cities, people find it challenging to navigate the crowds during rush hour; for example, it can take up to three hours for a person to drive a short distance on one of the Los Angeles freeways."
This is because the second sentence provides an example that illustrates the challenge mentioned in the first sentence.