When shooting in Light Trails mode (LT), it is advised to adjust the Shutter Speed (SS) and ISO - within the manual controls, on the main view - as needed, depending on the light conditions the shooter is in.
Most often, in order to achieve more linear and continuous trails of light, it is advised to go below 1/30 of SS value, or even below 1/15 and further down if needed.
This will ensure the shooter does not get the so called "morse code effect", which is when the trails of light are showing as shorter ad repeated trails, not providing the smooth and linear end result one is after.