The best transition word or phrase to clarify the relationship between the ideas in the sentences is "for example."
This transition indicates that the second sentence provides a specific instance that illustrates the general idea presented in the first sentence about the challenges of navigating crowds during rush hour. The sentence would read:
"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."