Mill Avenue

Many arterial roads in the Phoenix metropolitan area have the same name in multiple cities or towns. Some roads change names or route numbers across town borders, resulting in occasional confusion. For example, the road known as Apache Boulevard in Tempe continues east as Main Street in neighboring Mesa and then as Apache Trail in Apache Junction. Although Broadway Road maintains the same name through Goodyear, Avondale, Phoenix, Tempe, Mesa, and Apache Junction, each town uses a different reference point for address numbers.

Baseline Road is a significant east-west arterial road. This road is so named because it runs along the length of the primary baseline for Arizona as given under the Public Land Survey System.

This line runs east-west from the "zero point" atop a hill near the confluence of the Gila and Salt rivers, on Avondale Boulevard (115th Avenue) at Baseline Road. This site is next to Phoenix International Raceway. Avondale Boulevard is laid out atop the north-south base meridian, called the Gila and Salt River meridian.

Due to natural obstructions, this road is not continuous.The longest continuous stretch of the road is approximately 43 miles.The road is located in Maricopa County and Pinal County. There is also a rural road named Baseline Road in southern La Paz County close to the Colorado River which is located near Arizona's baseline.

Because of the grid plan used by cities in Arizona, the name for the road is applied for non-contiguous sections and is theoretically reserved for future east-west roads built at the same latitude.

...
Wikipedia