Riding between lanes of slow/stopped traffic. Debated but practical in some places.
Legal Where?
California: explicitly legal. CHP guidelines: no more than 10 mph faster than traffic, under 30 mph. Utah and Montana: filtering at stopped traffic under 15 mph. Most other US states: illegal or unaddressed. Legal in most of Europe and Asia.
Safety
Speed differential matters most. Stay within 10-15 mph of traffic. Between leftmost lanes safest. Watch for lane changers: signals, wheels turning, head checks. Be visible: headlight, hi-vis. Avoid blind spots: pass through quickly. Road surface between lanes often slippery.
When Not To
Rain. Intersections. Construction. Traffic at speed limit. If not comfortable.
Safety Argument
Stopped motorcyclists vulnerable to rear-end from distracted drivers. UC Berkeley: splitting riders less likely rear-ended.

