The best transition word or phrase to clarify the relationship between the ideas in the sentences is "therefore." This is because it indicates a cause-and-effect relationship: the challenges in navigating crowds during rush hour lead to lengthy travel times.
So, the combined sentence would be:
"In some big cities, people find it challenging to navigate the crowds during rush hour; therefore, it can take up to three hours for a person to drive a short distance on one of the Los Angeles freeways."