Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway.
The last time I saw that line, I decided to actually look into it.
In the 1970s, by which time the phrase was already in use, a typical station wagon would be something like an AMC Rebel. According to stats from Wikipedia, the Rebel has a cargo capacity of 91 cubic feet. For tape, the IBM 10.5" reel was the "defacto standard" from the 1950s "through the late 1980s". Assume 10.5"x10.5"x.5" i.e. 55.1 cubic-inch rectangular prisms as the tightest possible packing (which is optimistic, given that the tape itself is .5" not including the reels themselves, but the saying urges us to avoid undersetimation) 9-track tapes debuted in 1964, with densities of 800, 1600, or 6250 cpi corresponding to between 5 to 140 megabytes per standard 2400' tape. This gives us a capacity, then, of 2854 tapes per station wagon. At highest density (again, the phrase does urge not to underestimate) this corresponds to a whopping, in the mid-1970s at least, 390 gigabytes. I consider a trip from the University of Illinois at Urbana-Champaign to Norfolk, Virginia as a reasonable cross-country journey with computing-appropriate endpoints and a 55 mph 1970s speed limit. This is roughly 850 miles, managable in 15.5 hours with very quick stops for gas.
This finally corresponds to a mid-1970s bandwidth of 390 gigabytes per 15.5 hours, or in more familiar units, 57 mbps (yes, bits not Bytes, as is typical for bandwidth units).
The modern version would probably need to substitute an SUV for the station wagon. A 2012 Ford Explorer is listed at 81 cubic feet. Using common modern tapes, like jb/jx tapes, you could hold ~7000. In gen4 mode, these tapes hold 1.6 TB (yes, 4TB tapes exist but seem too extraordinary for this usage). At typical cross-country speed of 68 mph, the same trip would be 13 hours, padded by the same half-hour for gas the previous figure was.
The modern version, then, works out to some 11 petabytes per 13 hours, or something like 1900 gbps. This works out to a full terabyte transferred every 4.2 seconds.
Do not underestimate, indeed!