When an asphalt road ages, it usually cracks due to a combination of traffic wear and surface flexing with changes in temperature. Those of us who contend with ice and snow part of the year see these effects compounded by water freezing in the cracks and expanding. Without intervention, the cracks will grow into potholes which can damage vehicles. Eventually the potholes proliferate and widen, at which point the municipality needs to repair or replace the road surface. This is costly and disruptive.
In order to forestall this use of our tax dollars, the agency responsible for road safety usually acts when cracks become noticeable by applying a crack sealant. This is usually a rubbery, tar-based sealant which covers the crack on both sides and seeps down into it. The sealant acts as a glue to prevent more asphalt from chipping away. It's a good short-term solution which is non-disruptive to cars, trucks, buses and so forth. The long, wavy lines of tar that follow the cracks often look like flat snakes on the road.
Unfortunately two-wheel vehicles, like bicycles and motorcycles, but also in-line skates, scooters, and the like, can have trouble with tar snakes. Especially when the crack runs parallel to the line of motion, the tar snake can be a hazard in hot, cold, and wet conditions alike. Cold or wet tar snakes are slippery, providing little traction for cycle tires. This can cause a loss of balance if the rear wheel slides out, leading to a spill. Hot tar snakes become semi-molten and gooey, snatching at tires and guiding them into cracks where a cyclist or skater may be railroaded into an unwanted travel vector.
When encountering tar snakes, the cyclist or skater should avoid them if possible. If not, reduce speed, and try to cross the snakes at a 45 to 90 degree angle. In cold or wet weather, some experts advise that cyclists keep a loose grip when in a thicket of tar snakes, letting the bike slip across to find traction and balance, instead of using a death grip to force the bike onto one course.
I, myself, have had a front bike wheel grabbed by a molten tar snake and sucked into a crack just wide enough that the wheel lodged firmly in the asphalt. This event was followed by my sudden separation from the bike saddle and brief airborne experience. Then came the inevitable sudden stop. Fortunately it only cost me a new bike helmet, but of course a street crash can have much more severe consequences. Respect the tar snake and treat it with caution!