Yes you can drive in L all the time, though I choose not to do this for the reason that you can over decelerate sometimes and doing so and having to reaccelerate is less efficient. Also if you drive in L all the time (many people do this), you will get exceptionally high regen numbers in Braking Expert category as you are regenerating all the time (or alot of it) and really not an accurate braking score so to speak if you care about that.
It can affect the gas mileage yes by slowing down and speeding up more often. Best gas mileage is obtained by steady state speeds.
There IS NO BRAKE light on, so be careful with this strategy as you can get rear ended. In fact someone with a Cmax already did, saw pictures of that here, most likely because (just guessing) that the car slowed down with no lights and the person behind wasn't paying as much attention.
With all this being said, I drive in D all the time but shift to L on several occasions or situations:
a) slowing down to take a turn or getting off the highway. Basically when you are unsure of how much brake you can press as you need to slow down as fast as possible (without digging into the brake pads and lowering your braking score), shift to L first and then if not enough add some braking.
b) slowing down on the highway from 65mph to zero due to stopped traffic ahead. If no-one is close behind me I use L instead of braking on the highway to slow down as its very easy to overbrake on the highway and use the brake pads (dig into the brake score).
c) if a light suddenly turns yellow, same thing, need to brake as much as possible.
d) down a steep hill, or a hill that ends in a stop sign at the bottom.
e) late at night for almost all the stops if I am trying to travel 30 miles on a battery charge and need every ounce of power from the HVB to make it (due to headlights being on as they cut about 20mpge off the efficiency). Using L negates the brake lights coming on which for a 30 mile trip might gain me a 1/2 mile to a mile of range. Better to do that than start the engine for 1 mile at the end of the trip. When there is no-one on the road behind me the brake lights are not necessary. If on the other hand the trip is say 20 miles and I have plenty of battery to make it then I don't bother using L for all stops, just use the brake pedal and avoid shifting back and forth.
Be AWARE of your surroundings and any time there is someone behind you if you plan to use L just pad it ever so slightly with the brake pedal just enough so you start to see the circles going around the battery indicating that the brake lights are now on for the driver behind.
Hope this helps,
Edited by rbort, 20 October 2016 - 05:50 PM.