Two Dimensions, One Binding Answer

Kenya's speed framework has two overlapping dimensions, and you comply with both at once.

Reading them together — the binding limit at any point is the lowest applicable one, as posted. A car on a dual carriageway may run 110; the same car passing a signed school zone drops to 30; a lorry never exceeds 80 anywhere, regardless of what the road would permit a car. When a posted sign conflicts with your memorised default, the sign wins — signs are how the environment dimension reaches you.

Why heavy vehicles carry lower ceilings — this isn't bureaucratic tidiness, it's physics pointing three ways at once:

• Mass. Braking converts motion to heat, and the energy scales with mass as well as speed. A loaded lorry at 80 carries energy a car at 110 doesn't approach — through brakes that fade under sustained heat.
• Load behaviour. Cargo shifts forward under braking; passengers stand in buses. The gentler deceleration a lower speed allows is a load-security measure as much as a stopping one.
• Stability. Higher centers of gravity and articulation — the trailer's 65 ceiling — punish the swerve-and-brake combinations a car shrugs off.

So the ceilings equalize stopping capability across vehicle types sharing one road: a car at 110 and a lorry at 80 arrive at roughly comparable ability to respond to the same surprise.