減速 (gensoku) means “slowing down” | MotiTan Japanese Dictionary

More examples

見通しの悪い交差点では、早めの減速を心がけます。mi tō shi no waru i kō sa ten de wa haya me no gen soku o kokoro ga ke ma su

At intersections with poor visibility, I try to slow down early.

It indicates that the rate of movement decreases, while leaving the final state open.

徐行jokō: Movement by a vehicle at a very low speed for safety or caution.
スローダウンsurōdaun: A slowing of movement, progress, or activity.

進む速さが遅くなること。susu mu haya sa ga oso ku na ru ko to

減速 (gensoku) combines 減 (gen, "decrease") and 速 (soku, "speed") to mean a reduction in speed.

In road-safety notices, 減速 (gensoku) identifies a reduction in vehicle speed and does not by itself mean that the vehicle stops.