Actually the limit was 18 minutes an hour, not 16.
Most stations back at the time set 18 minutes as the maximum in any hour but in smaller markets and especially 'seasonal' markets, they hung to the idea of 18 minutes per hour on average for the whole broadcast day. Seasonal communities such as the ones found in Colorado, for example, barely billed anything outside the tourist season so they sold everything they could while the 'gettin' was good'.
For example, at one station I worked, we likely averaged 16 minutes an hour over a week. It might be 12 minutes an hour on Monday but 24 minutes on Fridays. After 9 or 10pm you might have 1 commercial. Now, one election day I was on the board, I had 5 minutes of network news and only managed to play 4 songs an hour...a good 40+ minutes an hour of ads. It was on par for the stations across the river from us in Mexico.
There was WDDQ in Adel, Georgia that was all commercials 58-59 minutes an hour.
NOW there is no limit on advertising in an hour/day/week.
On a related point, Underwriting Announcements, or what non-commercial radio calls their very limited form of advertising (let's call it what it really is) has no "Official" limits but the FCC does say when a station exceeds 6 announcements per hour or those announcements exceed 30 seconds in length per announcement, it sort of sets off red flags.