Currently the longest single-state mileage of any US route is 893 miles (US 83 in Texas). Historically, before US 101 was truncated to Los Angeles, that route was undoubtedly the record-holder: the California segment of US 101 was about 941 miles (give or take a few, depending on which year, since the alignment changed many times and in many places).
Historic US 99 in California was the second-longest route through a single state, although its mileage is even more difficult to measure, since it was replaced by I-5 in many areas, and as a result some segments of the original route have been vacated. The Historic Highway 99 Association of California offers a length of 917 miles. Although that page does not specify a year, I am sure that figure is reliable, since my own back-of-the-envelope calculation yielded a similar distance. (Note that drivers choosing US 99E between Sacramento and Red Bluff had to travel 2.2 miles further than those who chose US 99W.)
Thanks to this blog post, I recently became aware of "the Palm and the Pine" along what is now CA state hwy. 99, just southeast of Madera. In the median of the highway, a palm tree and a pine tree are planted side by side (one representing southern California, and the other representing northern California):
And why that particular location? The legend commonly told today states that the trees were planted at the midway point along US 99 in California -- that is, halfway between the Oregon border and the Mexico border. (US 99 no longer exists, but much of its historic corridor was re-designated as CA 99 in the 1960s.) So in other words, the implication is that these trees mark the point where one leaves SoCal and enters NorCal (or vice versa). But as I have noted before, the claimed midway points are not always the true midpoints, so I was interested in finding out whether or not this particular midpoint was accurate, and whether the legend could be true.
So the first question is: when were the original trees planted? US 99 was commissioned in 1926, so if the trees were planted prior to that, then we would know that their location could not have been based on the midway point of US 99. But unfortunately, not much information has been uncovered regarding the history of the trees. Other researchers have proposed dates that fall somewhere in the range of 1915 to 1928. So that leaves the question unanswered, although it does cast a little doubt on the idea that the trees were planted during the timeframe when the midpoint of US 99 in California could have been established.
But since we don't know for sure, let us proceed as if the location was indeed thought to be the midpoint of US 99. If so, then the question that follows is: how precisely were the trees placed? If we use the 917 mile figure cited above, the location would have been reasonably accurate, although it was not exact. 458.5 miles south of the Oregon border would put the midpoint right in the city of Fresno, about 11 miles south of the Palm and Pine. (The author of this blog post came to the same conclusion, despite using a different mileage figure for US 99.)
So, given what we know so far...
Where is the geographic center of California? Reportedly in 1996 it was determined to be near the small town of North Fork. This is in Madera County, not far from the Fresno County line. But back in the 1910s and '20s, determining the center of a state was much more of a manual process, often involving some guesswork. How would it have been done back then? Well, one could take a map of California (with an equal-area projection) and measure the distance from the state's northwest corner (near Brookings OR) to the southeasternmost extent (near Yuma AZ). The midpoint of that line would have been quite close to North Fork. Then, one could construct a perpendicular line bisecting the midpoint. That perpendicular line would intersect the main highway at a point between the cities of Madera and Fresno. I suspect that little tidbit of knowledge was enough to inspire a business owner (who had a store and rental cabins fronting the highway between Madera and Fresno) to plant some symbolic trees on his property, perhaps with the idea that the novelty might attract more customers.