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L
Lamps
Front and rear lamps are essential for riding in darkness or poor light.
Lanterne Rouge
The red flag draped over the road at the one kilometre to go stage of a road cycling race.
Locks
It is vital to lock up your bike securely with two good locks.

Use two different types of locks (one cable and one D) as thieves don't like to carry two different types of lock breaking tools.
Lowrider
Type of bike popular in US beach states, such as Florida and Southern California. They have fat tires, no gears and a low riding position which makes them fine to ride on the flat but would not survive long in the hills. Very popular in the 40s and 50s and now making something of a comeback.