Brake lights are a different kettle of fish than headlight modulators (probably should be a separate thread).
First, there is no standard for state statutes and they do vary widely. Flashing, modulating, strobe, all may have different meanings in those statutes. Many states talk about accessory lights vs. stock lights and most states will specifically address alternating between the stock brake lights and the white backup lights. Research your state and anywhere you may be riding to know the facts before making a decision.
Conversely, the MSF recommends that you pump your brakes for the sole purpose of flashing your stock lights. They also recommend that even after you stop, keep pumping until the cage behind you comes to a complete stop.
There are numerous options available. Generally they are actual flashers and not modulators (off/on as opposed to bright/dim). Some give the option to flash continuously or flash a number of times then go steady.
There are single wire units that you splice into the factory brake light wire. Existing brake light(s) flashes or flashes then steady.
For bikes with two or four brake lights, while more work, you can buy a unit that will toggle left to right and back. Sometimes called Wig-Wag style. If you have four brake lights you could also split and toggle one pair while leaving the other pair stock.
If you have added brake lights such as one in the spoiler, you can also insert the flasher unit into only those lights and leave the stock lights stock.
There are also motorcycle specific added lights that flash. SoCalMotoGear makes a replacement assembly for the Honda spoiler that has a built in flasher for the brake light. Hyperlites makes universal kits and a GL1800 specific kit that are available solid only, flash then go solid or even a flashing version that is California statute compliant.
Lastly, many folks have added emergency vehicle strobe lights from Whelan, Sound Off Signal, etc. The Whelan TIR3 is a common one as it directly fits in the location of the GL1800 rear reflector. These lights have multiple flash patterns and some offer the rapid flashes then steady feature. As a general statement, these are the brightest and most attention getting.
I have used many of the options. I have not performed scientific research so all I have is my own qualitative experience on which to offer my opinion.
After a Honda Civic tried to mount my GL1500 I purchased a Sound Off Signal six LED strobe and mounted it high on the backrest. This worked amazingly well. Eye level, very bright, rapid flash all would readily get the attention of tailgaters or routine traffic. I liked it so well I mounted one in a similar manner on my Valkyrie Interstate and my Mini Mate camper trailer. I may yet mount one on my GL1800 and/or new cargo trailer. I would recommend for anyone or any bike.
I have used the Hyperlites on two cruiser style bikes and my current cargo trailer. The small modules mount cleanly under the cross bar on the rear sissy bar luggage rack. They did not have a taillight function so when they came on it was like a car third high brake light and worked very well on these bikes, even with the small rear profile. Several positive comments from cage operators. No negatives specifically to the lights just one Prius once that continued to tailgate, even after I waved him around. He probably didn’t have enough horsepower to pass or was afraid the Valkyrie would suck him down the air filter like a bug. On the trailer they are lower but out of the direct sun. The trailer gets so much attention that I can’t clearly attribute all goodness to the Hyperlites alone. Also used their directional lights and they work great as well.
On the GL1800 the SCMG replacement spoiler brake light had not come out yet. I probably would have just gone with that. Instead I have the spoiler with the EC upgrade, the Sentinel lights from Add-On on the speaker boxes and the F6B harness to turn the lower center tail light into a tail and brake light. I installed the Signal Dynamics XP flasher on those lights and left the stock brake lights alone. I certainly notice people “Back Off” when I activate them. So far so good (knock on wood).
When it comes to lighting, I try to maintain the triangular pattern. GL1500 had the rear fender reflector replaced with the brake/tail light. Addition of the light on the backrest gave a triangle (four 1157 across the fender/bags, two in the trunk and the strobe on top. The Valkyrie Interstate was similar except two 1157 in trunk plus strobe on top. The Mini Mate, again, two steady Superbrite LED brake lights plus the higher strobe. The cruisers had an inverted triangle with stock brake and the two higher Hyperlites. The GL 1800 is also inverted with Sentinel and spoiler on top, narrower stock lights in trunk and single brake light between the directionals.
