Like much of the United States, driving is the preferred method of travel in California. The state's highway network is modern and extensive, but perennial budget shortfalls mean drivers regularly encounter potholes. While GPS and Google Maps are widely used, drivers often keep maps by Thomas Guide (now owned by Rand McNally) and AAA on hand just in case. In more remote regions of the state, such as along the Central Coast and in the Sierra Nevadas, AAA has more detailed and accurate information than almost any website in existence.

There are four types of highways in California: Interstate highways, U.S. highways, State and County Routes. All highways are designated with a number, and numbers are not duplicated. However, portions of highways may change designation. For example, Interstate 238 and State Route 238 both exist in the San Francisco Bay Area. Interstate 238 is the freeway portion of the highway, and State Route 238 is the surface street portion of the highway. Odd-numbered highways generally run north-south, and even-numbered highways generally run east-west. There are some notable exceptions to these guidelines, so plan ahead! Also there are certain highways and bridges that charge tolls, so you should plan ahead, especially with a Rental Car, more info can be found here. http://www.tripadvisor.com/Travel-g28...

California is enormous in size, and while it is known in popular culture for its good weather, actual weather conditions vary widely by location and time of year. Particularly in the mountains, highways can be closed seasonally or have extensive construction impacts due to landslides. The California Department of Transportation, known simply as Caltrans or CT, maintains an up-to-date database of highway conditions for all state highways. The database, which can be found here, also includes current construction activities that may impact travel. In the Southern California area, www.sigalert.com is an up to date traffic site that is run by Caltrans.

Traffic congestion is common in California's metropolitan areas, and regional 511 websites for Los Angeles, San Diego, and the San Francisco Bay Area report real-time traffic conditions. Traffic reports can be heard frequently on the radio; tune to KFI 640 AM and KNX 1070 AM in the Greater Los Angeles region, KOGO 600 AM in San Diego County and KCBS 740 AM (or 106.9 FM) in the Bay Area.

California is long north-south and has several primary highway corridors. The most common five are:

Interstate 5: I-5 runs from the U.S./Mexico border just south of San Diego to the Oregon state line and all the way to Canada. This is the primary route for travel between San Diego, Los Angeles, and Sacramento. Outside of urban areas, it is primarily two lanes in each direction with heavy truck traffic and limited traveler services. I-5 is a popular route for travel between Los Angeles and San Francisco, but it is substantially less scenic than Highways 1 and 101 and is used only when travel time is the ultimate consideration. However,  north of about Red Bluff, I-5 passes into the southern tip of the Cascade Range of mountains and is very scenic - as well as being subject to snowfall and extreme cold in winter.

U.S. Highway 101: U.S. 101 extends from Los Angeles to the Oregon state line and beyond all the way to Lund, British Columbia Canada. This is the main direct route for travel between Los Angeles and San Francisco, although I-5 is the route with that it is most used by cars and trucks (but as mentioned above, not very scenic). U.S. 101 runs directly along the coast in Southern California between Ventura and Gaviota before turning slightly inland. In northern California, Highway 101 is a pleasant, fairly scenic inland route to Eureka; it then travels along the ocean for much of the way to Oregon. The main highway goes right through Redwood National Park (no gates, no entry fees). Although only portions of U.S. 101 run along the coast, it is substantially more scenic overall than I-5. In San Diego County, there is the "Historic Route 101" which was part of the highway route, but got changed when Interstate 5 was built.  It is a scenic Coastal Drive along Northern San Diego County coast. http://www.sandiego.org/articles/tour...

U.S. Highway 99: Sometimes referred to as "old 99" and/or "Golden State Highway", this was one of the first 'through-roads' in California, dating from the 1920's. Today's modern freeway still follows the route of old Indian trails. The main section of 99 runs from just south of Bakersfield to Sacramento, and is an alternative to I-5. Since it passes alongside many towns, there are far more exits and traveler services along 99 than along I-5.

State Route 1: Highway 1 runs mostly along the coast from San Clemente (Orange County) to Leggett (Mendocino County). It is referred to as Pacific Coast Highway or simply PCH in Southern California, but it is generally known just as Highway 1 (and sometimes Cabrillo Highway) in Northern California. Highway 1 is a popular scenic highway connecting Southern California and the San Francisco Bay Area. Vast stretches of it are winding with one lane in each direction. It is a much slower but spectacular alternative to the freeways.

Interstate 15/U.S. Highway 395: I-15 extends from San Diego to the Nevada state line and beyond. It is a popular route to Las Vegas and Utah. Highway 395 connects with I-15 in Victorville (San Bernardino County) and extends north through the Eastern Sierra Nevadas to Reno, re-entering California to the north and continuing on to Oregon and beyond. Substantial portions of these highways run through the desert in Southern California as well as the Sierra Nevada mountains. U.S. 395 is a scenic inland highway and popular route for travel between Reno, Lake Tahoe, and Yosemite to the north and Death Valley and Las Vegas to the south.

 

A well known, but now mainly surface streets instead of a "highway" is Route 66, the California Portion goes from the Santa Monica Pier to Needles. http://en.wikipedia.org/wiki/U.S._Rou...

 

 

The California primary east-west highway corridors are:

 

Interstate 8: Runs from San Diego to Yuma, Arizona and Beyond, Stops include multiple Indian Casinos and El Centro.

Interstate 10: I-10 runs from the beach in Santa Monica east through Los Angeles to the California/Arizona border - all the way to Florida. Like I-5, stretches of I-10 are choked with heavy truck traffic until the eastern edges of the Los Angeles basin.

Interstate 40: I-40 runs from Barstow east to Arizona and beyond. It is a main route for travelers going between the Los Angeles area and the Grand Canyon.

Interstate 80: I-80 runs from San Francisco, across the Bay Bridge, and east to Sacramento, Truckee (near the north shore of Lake Tahoe), Reno, and beyond. Heavy snow fall occasionally shuts down I-80 around the infamous Donner Pass, but Caltrans strives to keep the freeway open all day every day, regardless of time of year.

U.S. Highway 50: U.S. 50 connects with I-80 in Sacramento and extends east to South Lake Tahoe and into Nevada. This highway is subject to heavy snowfall, but like I-80, Caltrans keeps this road open year-round.