According to Graham Allison in his book nuclear terrorism what percentage of the 50,000 daily cargo containers to the United States were screened

According to Graham Allison book, Nuclear Terrorism, only 5 percent of daily cargos into the U. S are screened where every day 30,000 trucks, 6500 rail cars and 140 ships deliver over 50,000 cargo containers into the United States. This rarely involves physical inspection which may not detect nuclear weapons and fissile materials.