The move is designed to make roads safer and encourage more walking and cycling
Nonsense, that’s not how to encourage other walking and cycling. To encourage walking you need to build pedestrian-friendly infrastructure where everything is in easy walking distance without crossing vehicle traffic. I cross roads four times with no crossing infrastructure in a less than 5 minute walk to my closest shop, that is not pedestrian-friendly by any stretch of the imagination. To encourage cycling we need proper cycling infrastructure, again without crossing vehicle traffic. That means proper cycle routes, not painted gutters barely as wide as a single bike. There’s a one-way bike lane near me which runs against the flow of vehicle traffic and both starts and ends in a high vehicle traffic road, I rarely see anyone on it for obvious reasons.
Changing a number on a sign does not change how people travel. If you want people to use other travel infrastructure that infrastructure must be the most attractive option for most journeys for most people. Getting hit by a car at 20 is better than 30, sure, but it’s objectively better to not risk getting hit by a car at all.