Water never breaks

How many of second or more time moms never had their water break or never lost their mucus plug until in labor and at the hospital? My doctor said only 20% of women actually have their water break on its own. This is my 3rd baby and I'm at. 38 weeks. I have an hour drive to the hospital and my last pregnancy was a 5 hour labor so I'm looking for any sign ahead of time. I'm 2 cm and 50% effaced as of yesterday.