According to the USGS report:
Fissure 8 lava fountains 52-70 m tall showered spatter onto the cone overnight into 19 June (figure 411). [...] Standing waves were visible within the channel and cascades/rapids were visible near the base of the 50-m-high cone. The maximum flow velocity in the channel was measured at 28 km/hour.
Now this report is really long and that statement is particular to one day's observation, so it's possible it was higher on some other day, but for most days only the height of the lava fountain is reported.
The eruption was filmed from a drone which referred to in the caption of an earlier figure (399) as the source of the lava flow measurement.
There's brief ground-level video posted by USGS in a montage (on Twitter) which bears some similarity with that one (but it's not exactly the same), with fairly similar commentary.
The lava flows out of fissure 8 and into a main channel, where the lava is moving up to 15 miles per hour, about the speed an average human can sprint over good ground. The flow slows down significantly before it reaches the ocean. [...]
“In this video taken from the Leilani Estates subdivision, lava at fissure 8 pulses above the cinder cone adding fragments of lava (spatter) that build the cone higher,” the USGS explains. “From fissure 8, lava flows freely over small cascades (rapids) into a well-established channel. Near the vent, lava is traveling about 24 km per hour (15 mi per hour). Lava slows to about 2 km per hour (1.5 mi per hour) near the ocean entry at Kapoho.” [...]
Late yesterday, the eruption vigor increased, and the rate of lava flow from fissure 8 ticked up significantly.
Standing waves visible in channel that moves 7.7 m/s (17 mph) near vent.
7.7 m/s is 27.72 km/h, so it's probably referring to the same event/measurement as the previous quote.
Now for "back of the napkin" verification... In the video from the question, there's a flying bit of lava that takes about one second (from 0:04 to 0:05) to cross the width of the (two-lane) road. According to Wikipedia:
In the United States, the Interstate Highway standards for the Interstate Highway System use a 12 ft (3.7 m) standard lane width, while narrower lanes are used on lower classification roads.
So, ignoring camera FoV/perspective issues, that's about 7.4 m/s for the flying bit.
Just so this doesn't get drawn out in the comments, there are numerous sources of error for this kind of estimate, including:
- actual width of the road
- actual time the blob is in the air (no millisecond timestamp on FB)
- FoV-related, i.e. distance from camera to end of the road vs distance from end of the road to the center of the flow, which the fast moving part. (The proportion of these two times the width of the road determines actual distance travelled by the lava blob.) You can see on the USGS drone video that this latter distance, from the end of the road to the center of the lava flow looks appreciable, e.g. relative to the size of houses. On the other hand, the road hardly appears to widen in the video in question, despite the camera being positioned on its axis, which suggests a pretty long telephoto lens being used, so the camera could be located pretty far off from the end of the road.
So, for these reasons, this my calculation can easily be off by a factor of 2 or even 4 in either direction.
Fast lava flows aren't too common but not unheard of. One in Africa (Mount Nyiragongo in 1977), was supposedly clocked at 40mph, which according to Guinness is [still] the world record on that matter.
Addendum. I cannot find anything too conclusive on this, but it's possible that the swirly motion
of the lava from the OP's video amplifies the subjective impression of linear speed, based on some experiments, which however found a limited subjective increase, around 10%. The hot points of the lava are quite bright relative to the background, which may also increase the subjective impression of speed; the latter paper also notes "substantial intersubject variation" in this regard.