The BBC News article The Rocket Scientists Mixing Up a Giant Firework begins with:
In a remote corner of tropical South American jungle, French scientists are mixing a ‘cake’ with a difference – a highly explosive rocket fuel for a new breed of space rockets.
and goes on to mostly talk about how to mix the ingredients and then bake them "like a cake" into a form ready to become an SRB.
Answers to If not constrained by underpasses, etc., would Falcon 9 have been less of a flying noodle? suggest that Falcon 9 rockets are driven down the road (on a truck of course) and this is confirmed by this answer, and photos of that happening are not uncommon:
- Why are these Falcon 9 1st stage bodies (apparently) wrapped in black plastic for transport?
- What support equipment is necessary to maintain a falcon 9 booster during transport after recovery?
Question: But what about SRBs and solid rocket motors? Are they ever transported on public highways? Or are they considered too dangerous for that? If not, does it mean these "cakes" are always "baked" in close proximity to their intended launch sites?
note: While even the segments of the largest SRBs are extremely heavy and that might be a practical limitation to highway transport, there are smaller ones, and solid rocket motors are even used as parts of payload spacecraft as well (example below). So let's not limit answers to only those that are too heavy for public highways, and instead keep the focus on the safety of transporting solid rockets on public highways.
below: Image of Orion 38 motor from Orbital ATK's Propulsion Products Catalog, borrowed from What inclination change could the 5th stage Orion 38 motor provide ORS-5 (SensorSat)?
Above could probably fit in a 1-ton pickup truck, but below is probably strictly off-road.
below: "Transporting the rocket boosters is a slow and steady process" (Credit: Esa) Source