At what speed do tall vehicles suddenly become transparent?
The idea that cars should drive past tall vehicles at 5 mph is slightly ridiculous. I have never ridden in a car with someone who constantly adjusted their driving speed based on the cars parked on the road. Choosing a reasonable speed? Of course. Extra care at obvious obstructions? Absolutely. Slowing traffic to 5mph because a van happens to be parked? No.
It is not ridiculous if you are driving in a narrow lane close to the obstacles. When I am driving on a residential (one lane, parked cars on both sides) street and I am approaching a tall opaque van I start by noticing if anyone is around it as I approach, when I have better sight lines, then as I get near it I absolutely slow down from an average speed of 15-20mph to well under 10mph and yes, depending on the situation sometimes as low as 5mph or slower. When I am on a faster road with two or more lanes there is often room to move laterally in these situations and so my speed reduction is less extreme, but I absolutely do still slow down as I pass these vans/trucks, cover the brakes and also check my blind spot to see if there is room for evasive maneuvers should they be required. If I see people in the area as I approach and have reason to believe they might try to cross, enter or exit a car, or otherwise be near the lane of traffic I will often change lanes to the left if possible.
The idea that cars should drive past tall vehicles at 5 mph is slightly ridiculous. I have never ridden in a car with someone who constantly adjusted their driving speed based on the cars parked on the road. Choosing a reasonable speed? Of course. Extra care at obvious obstructions? Absolutely. Slowing traffic to 5mph because a van happens to be parked? No.