The Texas Triangle is one of eleven Megaregions in the United
States. The term comes from the fact that the three main cities in the Texas Triangle are connected by a highway system Interstate 45, Interstate 10, and Interstate 35 that forms a triangle when connected.
The Texas Triangle contains 5 of the 20 biggest cities in the US, and is home to more than 70% of all Texans (pop 13.8 million).
The Triangle is anchored by the metropolitan areas of Houston, San Antonio, Dallas-Fort Worth, and Austin. Source:
Wikipedia: Texas Triangle.