I work out in that area and drive various parts of I-15 between Barstow and Stateline several times per week. There's a frontage road most of the way from Barstow until just past Harvard Rd. From there to Stateline there are no other alternate routes unless you go through Kelso. LA to Barstow you'll have to stay off the freeway until you pass Victorville. The shoulder is pretty wide for the entire route now but there's a sizeable rumble strip. The main issues to worry about are 1) lack of services- Victorville, Barstow, Baker, Stateline are your only reliable sources of water. Other places may or may not be open or in business on any given day. 2) road debris- there's literally tons of it on the shoulders, everything you can imagine (or not) will be found and a lot of it is bad for your tires. Take plenty of patches and a spare tire. (you won't find any bike repair/parts after Victorville and Barstow). 3) speed- the cars average 85-90 mph, the trucks will run 80+ on the flats and more downhill (I know because my firetruck max's out at 72), the air blast of a passing semi is amazing sometimes and there's often road debris thrown up. They may or may not move away from you but usually not (won't even move over for my warning lights). 4- weather- in January it can be 70's during the day but down in the low 20's at night and the wind almost always blows. The humidity will be less than 10% so dehydration will be a factor. 5- As Machka says...why? That area can be very pretty but I'd go through Kelso for better scenery and quieter roads, although those roads have less shoulder, and often no shoulder.
Good luck!
Sam