Spot speed study methods and rotary limitations

Spot speed

Spot speed is the instantaneous speed of a vehicle at a specified point (spot) of the road. It is used for speed limits, design speed, sight distance, signal timing, crash analysis and checking road geometry (85th-percentile speed is the usual regulatory speed).

Methods of spot speed study

1. Stopwatch (enoscope-based) method: a short base length (about 30 m for speeds below 40 km/h, 50-100 m for higher speeds) is marked; the time a vehicle takes to cross it is measured with a stopwatch, and $v = d/t$. Simple and cheap but less accurate; the enoscope (mirror box) reduces parallax error.
2. Pavement contact devices: pneumatic road tubes or piezo/induction loops across the lane record the passing of axles and give speed from the time between two sensors; automatic and accurate, good for long counts.
3. Radar (Doppler) speed meter / lidar gun: a hand-held or tripod unit measures speed directly using the Doppler effect; quick and accurate, but drivers may notice and slow.
4. Video and image processing: vehicles are recorded and speeds computed from frames and a known base length; gives permanent records and classification.
5. Automatic number-plate / GPS / floating car methods are used for long sections and for travel time.

Sample size is at least 30-50 vehicles, chosen at random; observations are made in off-peak free-flow periods under good weather (IRC:SP:19).

Advantages and limitations of a rotary

Advantages

• No signals needed; traffic flows continuously, so delay is low at moderate volumes.
• Crossing conflicts are replaced by weaving, merging and diverging, so crashes are fewer and less severe.
• Low speeds and low cost of operation and maintenance; no power needed.
• Suitable for junctions of four or more arms with balanced flows.

Limitations

• Needs a large area of land, so it is unsuitable in built-up areas.
• Not suitable for very high traffic (above about 3,000 PCU/h) or a very high share of weaving traffic.
• Pedestrians cross with difficulty and the detour adds distance to through and turning vehicles.
• Not suitable where one road is far more important than the other, or where slow vehicles are many.
• Congestion on one arm can lock up the whole rotary.