I have a product to heartily and door-handle-clutchingly recommend: reflective magnetic student driver signs.
These have SIGNIFICANTLY improved my feelings about sitting in the passenger seat while Rob drives. I feel like everyone around us is FOREWARNED (or REARWARNED or SIDEWARNED, as the case may be). And people DO give us more space. When I was researching student driver signs online, I found a BUNCH of people saying no, don’t use signs because there are people who will target student drivers and make things difficult for them: honking and zooming nearby to try to scare them, cutting them off, yelling taunts, etc. I don’t know what kind of gross pinehole would do such a thing, but so far we haven’t encountered ANYONE like that. So far what it seems to be doing is making other people more willing to let Rob merge, more tolerant of him going the speed limit or waiting too long to make a turn, more tolerant of his occasional slight weaving or over-braking or whatever. I am thinking I will leave the signs on even when it is ME driving.
Originally I’d made signs by using the largest font that would fit on a piece of printer paper, and then gluing that to cardboard:

Those blocked too much of the window, were hard to wedge into place, and weren’t much good at night. I still do have one of those in the front window on the passenger side, but then I use the magnets on the driver door, the front passenger door, and the back of the car. If the car looks dirty, I first take a half-wet washcloth and wipe/dry where I want to put the magnet. When we’re done driving for the day, I take them off the car and put them in a nice flat stack on the dining room table, which is where I keep everything I don’t know where else to put it.
Several reviews mentioned that the magnets can come flying off at high speeds, but we’ve done a couple hours’ worth of highway driving by now (did you mistake my new white hairs for highlights?) and haven’t lost one yet. It’s nice to be braced for it, though: I went into the purchase thinking, “Well, they’re about $6 each, and if we DO lose one from time to time, they’re STILL worth it as long as they’re not dropping off every time we go driving.” And they haven’t dropped off at all yet, so that’s good. If one did, I would have an order for more signs placed within microseconds.
