Some freeway stretches post a minimum speed as well as a maximum. What is the purpose of a minimum speed limit?

Why C is correct

Crash energy on high-speed roads depends heavily on speed differences, and a vehicle crawling at half the prevailing flow forces every vehicle behind to negotiate a moving obstacle. Minimum limits exist to keep that mismatch off roadways designed for continuous flow, where slowing vehicles have no driveway or shoulder to use instead. The corollary matters as much as the rule: a driver whose vehicle genuinely cannot sustain the minimum speed does not belong on that roadway at that time, and should use an alternative route with lower speeds and more exits.

Why the other options are wrong

A. To guarantee that emergency vehicles can always exceed it

Emergency vehicles exceed any limit under emergency conditions regardless of what the minimum is. The minimum is written for ordinary traffic flow, not as a head start for anyone.

B. To make space for overtaking by discouraging slow vehicles from the left lane

Lane choice is handled by keep-right rules, which are a separate instruction. The minimum limit is about who may use the roadway at all, not about which lane the permitted users should choose.

D. To ensure drivers save fuel by keeping speeds uniform

Uniform speed saves trivial fuel, but the minimum limit is a safety instrument. The target variable is the spread between vehicles, which is what turns a freeway into a series of closing-rate problems.

On the road

A loaded breakdown truck limps onto the interstate at half the flow, and within a kilometre three cars have nearly run into it — each one closing at a speed the road was engineered around, meeting an obstacle the road was not. The minimum limit is the law's way of saying what that scene shows: this roadway has a flow, and vehicles below it need a different road.

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