I think our area....Chicago and vacinity....was probably typical. WIND, which had a good signal, had what essentially a top-40 type format (without it being called that) since the early 1950s. Built around playing current hits. When rock and roll emerged in the mid-decade, WIND kept essentially the same format as it had earlier, but relegated Chuck Berry, Little Richard, and the harder Elvis stuff, etc. to nighttime. During the day, you'd get Elvis' softer tunes (e.g. "Don't be Cruel), along with Fats Domino, Everly Brothers etc., mixed in with Sinatra, Peggy Lee, Guy Mitchell, etc.
After a year or two of this, WJJD, an daytimer with slightly extended hours and a signal inferior to WIND's launched a more traditional top-40 format which included both the harder AND softer stuff. So you could hear "Volare" and "Charlie Brown" in the same half hour. 'JJD held its own with this for a while until WLS put basically the same format on a fulltime 50KW non-directional blowtorch signal. As for WIND, their format stayed fairly similar, but they gradually ditched the harder stuff at night. By the early and mid-60s, WIND's music strongly reflected the Billboard Magazine "Adult Contemporary" Chart.