I can't help but notice that many people posting to this thread do not live in Southern California and probably have never ridden extensively in San Diego. I do and I have. In the past, I've lived and worked in San Diego for more than 10 years in different locations, including Oceanside, San Marcos, Carlsbad, Encinitas, Rancho Bernardo, Poway, Mission Valley and downtown. I've ridden on every interstate and nearly every state route in the area.
Freeway/highway construction varies widely by location and what you think you know about your local road surfaces may not apply very well at all to another area. San Diego freeways make extensive use of jointed concrete with expansion joints at regular intervals that are not entirely level, and the surface has longitudinal grooves that are cut with diamond blades. This has a substantial effect on all vehicles, but particularly motorcycles and scooters. These grooves and expansion joints can be much more challenging for scooters, particularly lighter ones with short wheel bases and smaller wheels. The wheels tend to follow the grooves. The expansion joints cause a very regular vertical bounce and vibration that can turn the dampening of shocks into bounce amplifiers. It's very much like riding on a bridge all the time, except on concrete instead of metal or asphalt, and at 75 mph just to keep up with the flow.
The local traffic conditions are also substantially different than elsewhere. San Diego has some of the highest average speeds when traffic is clear, combined with very slow bumper-to-bumper congestion for miles with no alternate routes. The geography plays a role here because the I-5 borders the ocean and crosses lagoons that are bottlenecks. The I-15 also has bottlenecks due to lakes, canyons and hills. The result is that traffic flow can be well over 75 mph with many people passing you at 85+, and you can suddenly hit bumper-to-bumper traffic at a stand still that isn't visible until you're right on top of it.
You can be parked on the shoulder in a car more than 15 feet away from the nearest lane of traffic and the whole vehicle shakes hard due to the air pressure of large vehicles traveling faster than 80 mph. Smaller scooters can be very unstable when hit by these pressure waves.
So yes, it's important to have good tires. But that isn't necessarily going to make a GTS feel stable and safe on these freeways. You might be able to get used to it, but that's pretty hard to do when you have all these factors working against you. Most scooter riders do not brave the freeways unless they have a maxiscooter capable of doing 85+mph, and even then they are rare for commuting compared to motorcycles of the sport and sport-touring class. San Diego has lots of individual communities where smaller scooters work well inside that limited area, but commuting from one area to another on the freeway is a very different challenge.
As I write this, there are 15 sensors on I-15 that are recording average speeds above 70mph and many of those are pegged at 75mph, the max that is reported, meaning that the average could be substantially higher than that. You can see the
real-time traffic speeds for yourself. Would you feel safe on a GTS with an 80mph average flow of traffic with people passing you constantly at 85-95mph in SUV's? Now add grooves and expansion joints to that.