There's definitely a causative relationship between A and B, although it's not the only factor (of course, safer road design is probably a more significant factor)

When mandatory helmet laws were introduced across Australia we saw a sharp drop off in casual cycling rates which we're only recovering from now. If we never suffered that drop, perhaps we'd have a stronger cycling culture today.

Mandatory helmets are one of the worst things that a county can do for cycling.

Mandatory helmet usage reduces ridership. The inverse is not necessarily true.