Traffic measurements – use of an enoscope An enoscope (periscope-type roadside device) is primarily used to measure which traffic parameter?

Introduction / Context:
Traffic engineers use a variety of instruments to measure speed. An enoscope is a simple optical device used from the roadside to observe and time vehicles over a short base, yielding instantaneous or spot speeds.

Given Data / Assumptions:

• Known base length on pavement (e.g., marked on road).
• Observer times a vehicle passing two reference points using an enoscope.
• Short observation interval → instantaneous (spot) speed.

Concept / Approach:
Spot speed is the speed of a vehicle at a particular location and instant. By measuring the time to traverse a small, known distance and using speed = distance / time, spot speed is obtained. Averages over long sections or travel times require different methods (floating car runs, license-plate matching, GPS), not an enoscope.

Step-by-Step Solution:

Verification / Alternative check:
Repeated spot speed checks produce a distribution (histogram); mean/85th percentile values can be derived for design and enforcement studies.

Why Other Options Are Wrong:
Average speed over a length requires journey time methods; travel time is not directly measured with a short-base periscope; occupancy uses sensors/cameras.

Common Pitfalls:
Too short a base increases timing error; parallax or misalignment; failing to account for grade and curvature if comparing sites.

Final Answer:
Spot speed of vehicles