Leading Indicators

There's a bunch of things that provide useful information to a rider in advance of encountering an obstacle or potential hazard, which I will lump under the heading "Leading Indicators"--things that warn you of what's coming up.

1. Those dark splotches across a lane. Those are accumulated oil droplets that have flung off the undercarriages of countless vehicles as they bottom out following a ridge or rise in the pavement. You should expect a decent-sized bump about three meters ahead of the splotch, and maybe get up on your foot pegs on the balls of your feet to absorb the shock. Also be aware there's probably enough collected grease and oil there to be a little treacherous when rain starts to fall.
2. Wind-blown debris. Leaves, dust, dirt and debris make the wind visible; if there's enough of a wind to start lifting little things, there's enough of a wind to alter your trajectory, so be prepared. Be especially wary of 'dust devil' winds, which will hit you from both sides in rapid succession. They can be disorienting, to say the least. Besides being leading indicators of rapidly changing riding conditions, they are warnings to put your visor down to keep particles out of your eyes. Nothing worse than being momentarily blinded and tossed around. 
3. "Guilt Lights." Drivers ahead of you tapping their brakes reflexively when seeing a speed trap. I've never ridden through a speed trap that wasn't telegraphed by the drivers ahead of me in one form or another. It always makes me laugh but I am also more than happy to let someone else 'rabbit' for me.
4. Pedestrian Crossing lights. The countdowns running parallel to you let you see exactly how long you have before your light is going to change. No sense in braking prematurely, is there?
5. Birds, particularly crows and vultures. Obviously warning of carrion ahead, a potential road hazard. But frequently a hazard in themselves, particularly vultures, who gain altitude slowly enough to be a frequent risk to riders. When you see crows ahead, think vultures. And slow down.
6. Chasing your shadow. This is a leading indicator for others, mostly. If you're riding into your shadow, that means you're obscured by glare; the longer and more defined your shadow is, the worse the situation is for you. Slow down, turn on your high beam, get all big in the saddle (primate aggressive display behavior) stand up on your pegs if need be. Just remember that while you're always invisible, you're extra special invisible with the sun dead behind you.
7. Smells. Smells are a weird kind of leading indicator, because they are ephemeral, vague and poorly defined. There are really only a few that matter in that they convey information about your immediate environment. And in my experience, you rarely get useful smell information about your own vehicle, unless something is really, really, critically bad. But in general: 

• Diesel fuel/kerosene smell: Be aware of spilled fuel underfoot; slip hazard.
• "Maple Syrup:" Spilled or leaking coolant; also a slip hazard. Could be you if you're riding one of those fancy liquid cooled gizmos.
•  Burning brake pads or rubber...sometimes you'll smell a backup or sudden stop before you see it.
• Gasoline: Rarely an issue, because it's not as slippery as diesel and evaporates almost immediately. But if it comes on strong, fast and intense, check for a burst injector line/fuel leak ASAP. 
•  Grease/Gear oil-garlicky and unpleasant. Not very common but be aware.

To isolate the source of a particular smell, change lanes, speed up or slow down. You can frequently identify the offender in a few seconds, and take appropriate action.