Packing a Lot into Twenty-Four Hours
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Twenty-four hours is quite a lot of time for a drive that normally takes four to five hours. and so you have a huge opportunity to wander off the straight shot motorway (I-15) and see a bit of the desert Southwest. If that suits your plan, then look at leaving San Diego eastbound on I-8 to CA-79 north and CA-78 east through Anza-Borrego Desert State Park. From Borrego Springs take county route S-22 to CA-86 north along the shore of the Salton Sea (226 feet below sea level). At Mecca, follow CA-195 (66th Ave) to Box Canyon Road to I-10 east and then CA-177 north and CA-62 east to Parker, AZ. From there, follow AZ-95 north through Buckskin Mountain State Park and Bill Williams River National Wildlife Refuge to Lake Havasu City and an ersatz taste of home. Finish up by continuing north to I-40 west to US-95 north to Las Vegas.
Such a route is about 50% longer than the motorway route, but is an entirely different experience. It will take about twice as long to drive, due to the nature of the roads and the extra distance, but even with a 10 hour stopover for the night still leaves you about 5-6 hours to get out of the car and explore some of the sights along the way and still arrive in Las Vegas 24 hours after your departure from San Diego. Where to spend the night would depend on what time of day you leave San Diego, but there are plenty of options en route.
AZBuck