塞 has three pronunciations: sāi (to stuff/plug), sài (frontier fortress, as in 要塞 yàosài), and sè (same as sāi but more literary). The sāi reading is the most common.
usage
塞车 (sāichē) means 'traffic jam' and is commonly used in southern China and Hong Kong. In northern China, 堵车 (dǔchē) is more common.
