OK, some info first.

My commute is a mere 5 miles, with a lot of traffic stops (17 stop signs, 8 traffic lights), but not so many that I can't speed along, if I feel like it.

At 5 miles, it is a nice jaunt, which I typically complete in 19 to 23 minutes, depending on what lights I hit.
I figure my short commute would be a good way to pack in a little more exercising by ramping up the intensity.
I could change my route to be more challenging, or bike longer, but that'd take extra time, and I barely get to work on time as it is

I am familiar with HIIT in terms of calisthetics, but not sure how to translate that into cycling.
I imagine this is mostly a wattage game, but not sure if I am better served increasing cadence, or increasing force?
I currently cycle around 80-90 RPM (best estimate, as I don't have a computer to measure it)

I am leaning towards higher cadence, as I feel higher force = mashing = spontaneous knee explosions

Given the amount of stop-and-go, would learning how to sprint be a good way to crank up intensity?

Sure, why not?
Do both : high cadence days, low cadence days.
High intensity days
Low intensity days for recovery; especially after low cadence days.

High cadence will tend to work on cardiovascular while low will tend on muscle strength. Both are important but being able to spin fast is the gateway to fast acceleration. Going from 90 to 120 is a huge increase in wattage and happens really fast. You can also spin much higher for a short while before shifting up and settle on a steady cadence.

The simplest way (although not necessarily the safest way) to incorporate HIIT into your commute is to sprint for every light.

You could also do something like 30s on/off intervals. Those don't require power or cadence, just the timer function on your computer or watch. Again, very simple. 30 seconds on means full gas, as hard as you can go, for the full 30". Then recover for 30" by coasting or soft-pedaling. The problem with strict time intervals is that they often don't work well on commutes where you have lots of stop signs and stop lights.

Ah yeah forgot to say : safety first
Don't do that on crowded streets (ie: downtown Montreal)

I do this one trip a week. The trick is to pick stops that are far enough apart that you can safely pedal all out for 30-60 seconds at high resistance and still have 60 seconds to recover while slowly spinning. I have 4-5 intersections, plus a few long stretches of MUP depending on which route I use. I get about 8 intervals.

It's probably suboptimal from a HIIT viewpoint, but better than not doing it.

commuting can be a good opportunity to do intervals. You can mix it up a bit.

Practice hard accelerations from a stop
practice shorter high gear accelerations to build muscle
practice longer high cadence spins to give yourself speed.

Above all, make sure you are in a safe place if you are going to redline you heart rate or push yourself. For that reason, I would concentrate more on effort rather than on speed while commuting.

Thanks everyone! This really helps with a starting point

I like the idea of sprinting from stops, and going lower gears/high cacdence for 30ish seconds or so, coupled with other days working high gear/low cadence (until I am forced to stop)

Without strict timing (traffic, and lack of computer prevent this) it will be suboptimal. But as SloButWide mentioned it's better than not doing it.

DunderXIII, I do not know how you hack Montreal roads. Hats off to you, sir

You can break the standing starts down too:

1) small ring (39x15): focus on spinning as quick as you can at top end
2) big ring (52x14 or 15): super high force to start so make sure your bars are tight and your chain/cassette don't skip; focus on getting on top of the gear

Thanks. This is actually very similar to my standard cycling configuration
Normally I accelerate from stop using 39x15
Then switch to 52x15 to cruise (my cassette is 11 - 13 - 15 - 18 - 21 - 24 - 28)

So, using your recommendation, all I'd need to do is take turns on the small and big ring at each stop.
I like it! Less thinking for me. Gonna give that a shot

I'll only use 11 or 13 when I have a decent flat or downhill stretch of road ahead (i.e green lights)
I only use 18 - 21 - 24 on climbs which I rarely have (average elevation gain is only 33m)
I don't think I have ever used 28.