It depends on map you are using.
If the map you are using has Speed Limit data, your SLI becomes red when you are over the limit and you hear a sound (if you've enabled "Audible Speed Alerts" in Settings>Map & Vechicle>)
If the map doesn't have Speed Limit, nothing happens.
During Route Simulation you can increase the speed of the car by touching the Speed Limit icon and changing the speed number.
When you reach a road with a different Speed Limit (map data) than the one you've set, your Speed Limit will be restored to the map's value and you will have to change it again.