Our students have tremendous obligations. If students already know the material, don't waste their time by expecting them to be in class. If your course is structured like an independent study, meaning all the information a student needs is in the textbook, attendance could be optional. A lot of tardies add up to an absence. Tardiness creates the problem of having to repeating directions.
Some students leave class early in order to avoid the parking lot mess.
Organize classes with a lab or workshop at the end of the class session so students can choose to stay late or leave early. If the instructor doesn't show in the first fifteen minutes you can assume class is canceled. Attendance is crucial for success in this course. Your grade depends on class performance and the number of hours you are in class. Consistent attendance and participation are critical to successful completion of this course.