Many people set their speed by the legal limits imposed upon them. If they are in a 30 MPH zone, that’s the speed they’ll aim to travel at. If they move to a 50 MPH zone they’ll aim to hit 50 as soon as they are in it.
While this is often OK, it certainly is not always so. Far better is to drive at a speed appropriate to the conditions, making sure you don’t go over the legal limit.
Sometimes the legal limit is too fast
Traveling at 30 in a 30 MPH zone may be fine for much of the day. However, it may be much too fast around school entry or exit time, with parents pulling in and out of parking spots and children walking along the sidewalk and crossing the road. There is too much happening for you to safely take it all in and react if needed when going at that speed. Slowing down will give you more time and reduce the chance of a crash if you or someone else makes a mistake, which distracted parents and children could well do.
The same would apply in city center traffic that is grinding to a standstill. Driving at 30 MPH would lead to you crashing straight into someone within a few feet.
Both those examples are relatively obvious. Most drivers would automatically slow down in such situations. Yet there are plenty of other less obvious situations where you may need to go slower than the legal limit.
Poor road surface conditions
A summer thunderstorm could make visibility appalling and leave dangerous puddles on the road. Drivers who drive at the speed limit may struggle to see what is ahead of them as the rain beats on their windshield. If they hit a puddle they could send plumes of spray up that obscure the vision of other drivers. They might even hydroplane on the standing water and veer across the lane.
Once winter hits, those who head out early in the morning could encounter patches of ice that have not yet been gritted or melted away by the heat of passing traffic. Slowing down would reduce the chance they lose control.
If a driver injures you, they may claim they were not doing anything wrong because they were traveling at the speed limit. Showing they were traveling too fast for the conditions could help you get the compensation you need.