What is the basic speed law?

Two kinds of speed limit

• Absolute limits — exceeding the posted number is the violation, full stop. Most states work this way.
• Prima facie limits — the posted number is presumed safe, but a driver may argue that a higher speed was reasonable in the circumstances. California, Texas and several other states use this for some roads.

The basic speed law sits on top of both: even within an absolute limit, unreasonable speed for the conditions is itself an offence.

What «conditions» means

Weather and visibility, road surface, curves and grade, traffic density, pedestrians and cyclists, the vehicle's load and brakes, and whether it is a school zone or a construction zone. Doubled fines in work zones and school zones are standard in most states.

How these citations are contested

Because the standard is reasonableness rather than a number, the defence is factual: what the conditions actually were, what other traffic was doing, and how the officer measured or estimated speed. Where radar or lidar was used, calibration and training records are the usual line of attack. Where the citation rests on opinion, contemporaneous evidence — dash-cam footage, weather records, photographs of the scene — carries the argument.

Why it is worth contesting

The fine is rarely the real cost. Points, insurance surcharges lasting three to five years, and licence consequences for commercial drivers usually exceed the ticket by a wide margin. Many courts offer traffic school or deferred disposition for a first offence, which keeps points off the record.