What Are Those Squiggly Black Lines On Roads?

Another great mystery of the universe solved.


Tom Hale

Tom is a writer in London with a Master's degree in Journalism whose editorial work covers anything from health and the environment to technology and archaeology.

Senior Journalist

An empty road runs across the desert in California covered in black tar lines.
What are these so-called "tar snakes"? Image credit: Sarnia/

If you’ve ever taken a long road trip, there’s a strong chance you’ll have seen those black squiggly lines that cake certain portions of the highway. While these road scars may look messy, they’re typically nothing to worry about. The black lines are simply crack sealing that has been used to patch up the road.

The vast majority of modern roads in the US are paved with asphalt, which is essentially a sticky, black, semi-solid form of petroleum. Cracks can appear in this material for all kinds of reasons, from wear-and-tear or repeated traffic running over it to changes in temperatures causing the material to expand and contract. Water entering the cracks can also cause big trouble for the road surface.


Among the many different types of cracking, fatigue cracking can often cause the cracks to emerge and spread out in a squiggly, seemingly random-shaped pattern that looks a bit like an alligator’s skin.

Workers apply liquid tar sealant to cracked road.
A tar snake is born. Image credit: ungvar/

To fix the unsightly cracks, road repair workers will often use asphalt rubber sealants with added polymers to give more flexibility. The result is a tar-like liquid that is poured over the cracks where it seeps in and sets solid. 

This is done instead of repaving the whole patch of highway, or even overlaying its surface, which will prove very costly for city authorities. As per an old document by Texas A&M University for the state's department of transportation, it costs over $60,000 to overlay a mile of highway, but just $2,500 per mile for crack sealing. Since those figures came from 2008, the price tag is likely to be way higher today, but you can get the picture of how different the cost is. 

While most drivers will encounter no problems cruising over the crack sealant, some motorists believe they can be dangerous. Dubbing the pattern “tar snakes”, they argue that the sealant material can heat up on hot days, becoming more liquid and increasingly slippery. This can be an especially prominent problem for motorbikes, which are more prone to slipping on surfaces.


If you see a plain of black squiggles approach you on your journey, then there's no need to panic, although some will advise you to slightly drop your speed and keep your attention sharp.


  • tag
  • Engineering,

  • road,

  • crack,

  • asphalt