Listing every number shouldn't be encouraged, and practiced. It's bad because imperial speed limits can use 5mph steps, causing an even longer list than metric 10km/h steps. That's described as the "majority" in USA, although the case studies hasn't mentioned yet. The `maxpseed:advisory:variable=` can be again lower, as low as 25km/h = 15mph. https://highways.dot.gov/safety/speed-management/guidelines-use-variable-speed-limit-systems-wet-weather/chapter-3 
"Advisory speeds posted to the overhead signs change by no more than five miles per hour (MPH) with each change in speed, and can be updated every thirty seconds if traffic conditions warrant.  The minimum advisory speed displayed is thirty MPH and the maximum advisory speed displayed is fifty MPH.  If the current speeds on the roadway are below thirty MPH the signs will go blank. " https://ops.fhwa.dot.gov/publications/fhwahop12046/rwm17_minnesota1.htm
"When reductions are
warranted, VSL sign assemblies are manually activated to decrease speed limits in five-mph
increments from 50, 55, or 65 mph (80.4, 88.4, or 104.5 kph) to 30 mph (48.2 kph) depending
on prevailing conditions. " https://ops.fhwa.dot.gov/weather/Publications/Case%20Studies/14.pdf
Do we want  `=30 mph;35 mph;40 mph;45 mph;50 mph;55 mph;60 mph;65 mph;70 mph;75 mph` / `=50;60;70;80;90;100;120` ? That's certainly not impossible. 
 As you may know, `interval=` is already used for service frequency in terms of time. It's possible VSL can be reviewed and changed in some time periods, so that it won't fluctuate suddenly. Similar to congestion-responsive dynamic tolling, or transitioning between time periods in variable tolls (something that has happened locally this year, while I'm not very interested in `charge=` yet). 
What's "regular intervals"? "Interpolation" simply means how you should obtain the actual number. It doesn't mean it's exactly the same as how it works in `addr:interpolation=` . But `*:interpolation` is already used there. 
The term used often as I read is "increment". But that's overlapping as used in "to indicate the value to increment between house numbers" https://wiki.openstreetmap.org/wiki/Addresses#Interpolation