Why is it that between north of Burbank and all the way to Stockton there is not a single major town or city along the Interstate 5 all the way through the Central Valley? There are big cities like Bakersfield and Fresno along Highway 99 but why none along I-5?
Highway 99 is a state highway, built 1928.
Highway 5 is a US Interstate, built in 1957.
Major cities were pretty much established well before 5 came into existence.
That doesn’t really answer the question, though… In many parts of the U.S., the Interstate system was “built” by upgrading existing roads to Interstate standards. Why didn’t the California highway planners upgrade the extant Highway 99, which gets closer to more people, rather than building an entirely new highway? (Or does the current route also follow an older highway?)
Have you ever driven that particular stretch of 5? Who on earth would anyone want to live there? There are scattered ranches and farms so technically someone lives there, but there is no “industry” to create jobs that would attract large numbers of people to move there. The weather in the summer is hot and dusty, especially compared to the coast, and it’s not really a pretty place to settle.
The highway was placed there because it made sense to put it there and had nothing to do with attracting people to leave there. To see why they put it there just look on a map. Highway 99 was an old highway and meanders around. Highway 5 is an almost straight line running north and south. Building a new highway made more sense than straightening 99… assuming that was even an option at the time.
Presumably, there would be less traffic problems being away from major cities.
I do believe that the low-lying, dry valley would provide much greater ease of construction at first, less upkeep later, and a faster, straighter drive almost all the time.
Yeah, I-5 is a straight shot all the way up. It’s fast, you never have to stop. That’s the point. On 99, in every town, even if the highway is built up and modernized (instead of still going down Main Street), it’s got lots of traffic through every city, it winds around, it’s much slower. It goes through lots of built-up areas and fields which were already in use when the state wanted to build something fast and straight.
And you avoid stupid things like the speed limit being lowered 10 mph because you’re driving through the middle of a city (such as I-5 through Medford).
There is some desire to upgrade 99 and make it an Interstate : Interstate 9. That would only cover between the Grapevine and Stockton. I-5 did use the then US-99 route south of there. North of Stockton/Sacramento 99 is less likely to be easily upgraded.
But in nearly every other area of the country, the Interstate system was built to unite major cities, not pass them by. A quick look at Wikipedia’s list of US cities by population suggests that the only cities in the top 100 that are not on an interstate are the ones bypassed by I-5: Fresno, Stockton, Bakersfield, and Modesto.
So the arguments about why one might bypass a city are somewhat beside the point. The real question would be why those arguments were given controlling weight only in central California and nowhere else.
And I5 does connect major cities: Los Angeles and San Francisco.
I think the answer to the OP is that the older highways were only slight improvements over local-to-local connections that were originally dirt trails or stagecoach routes. Most auto drivers ca. 1928 didn’t drive long distances as the roads weren’t that good, and the roads weren’t good because no one drove long distances.
The Interstate system was built on a different premise. High-speed, long distance, connecting big areas, not small towns. And since the land had to be acquired by eminent domain, building a highway in the middle of nowhere is cheaper than tearing down a city or going thru the suburbs.
The good thing about it all is we now have two systems, each with their own strengths. Use the one that best suits you.
It is indeed very convenient to have 2 parallel but different highways in California, though driving I-5 always makes me want to take a giant tuck in CA between, say, Stockton and Bakersfield. No one would ever notice!
But I-5 was built for long-distance trucking between major cities, not connecting to every dinky town in the central valley.
You don’t say? 
I think people forget that 99 even exists north of Sac–in many places it’s still a 2 lane road. Several years ago CA news outlets made a big fuss about how 99 no longer had any traffic lights at all and had been fully freeway-ified. (Except north of Sacramento, where there are lots and lots of traffic lights.)
Another reason likely was cost. If you build you have to buy land or use eminent domain to get it. If you sieze land you’re gonna have lawsuits to slow you down. So it’s easier to build somewhere else, not to mention cheaper.
The OP questions seems to be, why build new instead of upgrading. This is probably part of the reason. Interstates need to meet certain specifications and it was likely easier to build around than try to sieze land or buy land to upgrade.
This is not a single incident though. For instance I-75 goes east of Fort Myers, Florida. The SW Florida area is the fasting growing but I-75 isn’t exactly close to Cape Coral, Bonita Springs, Fort Myers or Naples. Most people still use the state roads. But if you’re just zooming along from Tampa to Miami, I-75 is very quick. The Tamiami Trail, while prettier, is highly congested as it runs THROUGH the towns.
Also remember in the 50s, Bakersfield and Fresno were not nearly as big as they are today. It was probably deemed easier to bypass them and connect them with a spur, to I-5 rather than a real need to go near those towns. Of course today Fresno with 500,000 people is the largest city with no direct connection to an interstate. In 1950 Fresno had only 91,000 people
In 1950, CA had 10 million instead of 33 million, so while Fresno has grown faster than the state as a whole, it’s not by an astonishing margin - 5x as opposed to 3x. In 1950, it was not insignificant as a population center. One wonders about Bakersfield. Ok, the central valley route goes completely west of Fresno, but they seemingly bypassed Bakersfield just to be inconvenient. Had they followed 99 north for another 20 miles or so, they could have given 58 a junction with the interstate, then swung west and followed the present route if they wanted. If you’ve ever wanted to take 58 East (to go to Las Vegas or Flagstaff, for instance), you know that Bakersfield is one enormous PITA to get through.
I-5 was built to handle traffic between Southern California and Northern California. Highway 99 is already there to handle the traffic going to the valley cities themselves. It may not be up to interstate standards, but much of it is a freeway.
Not true. Look at I-80. It bypasses Cleveland, Chicago, Des Moines and Omaha. At Memphis, I-40 goes north, I-55 lies west and south. The point of the Interstates was to get from A to B – not to worry about connecting every place between.
I’m not sure in what sense I-80 “bypasses” Des Moines and Omaha; you’d be hard-pressed to argue that it doesn’t serve those cities, even if it doesn’t go right through their downtowns. As far as Cleveland and Chicago, they’re still connected to the Interstate system via I-90 (twinned with I-94 through the bulk of the Chicago area), so they’re still connected to “the system”. If there was another interstate that did serve Fresno, Bakersfield, et al., then the analogy might be more apt.
I-80 bypasses those cities only for very low values of “bypass.” 
And it connects Toledo and South Bend.
Agreed. The Des Moines downtown area is bypassed but it definitely goes “through town” for about ten exits.
It seems to have been placed as to serve local traffic, not just long-distance traffic.
If all I-80 were supposed to do was connect Cleveland and Chicago, they would have built it from downtown to downtown. Instead, it was built to bypass those cities (the cities eventually grew out to the Interstate, but that came later.) Ditto with Des Moines, where it was built north of the city limits, and Omaha, where what is now known as I-680 was originally I-80.