Test: Spot Speed Study - Civil Engineering (CE) MCQ

Spot speed studies are conducted to document the distribution of vehicle speeds as they pass a cross-section of the highway. Spot speed is the instantaneous speed of a vehicle at a location.

Spot Speed Study

• Spot speed studies are used to determine the speed distribution of a traffic stream at a specific location. The data gathered in spot speed studies are used to determine vehicle speed percentiles, which are useful in making many speed-related decisions.

Spot speed studies have a number of safety application as follows

• Speed trends
• Traffic control planning
• Accidental analysis
• Geometric design
• Research studies 

Additional Information
Enoscope Method

• It consists of a simple open housing containing a mirror mounted on a tripod at the side of the road in such a way that an observer's line of sight turns through 90°. It is an easy, economical but slow method.

On a freeway, spot speed studies are not conducted if the traffic volumes exceed 750-1000 veh/hr/ln. On an uninterrupted flow facility, the spot speed studies are not conducted if the traffic volumes exceed 500 veh/hr/ln.

The 98^th percentile speed is adopted for the geometric design of highways.

Geometrical design speed = 98^th percentile speed
Safe speed limit = 85^th percentile speed
Minimum speed limit = 15^th percentile speed
From the data given in the problem, 98th percentile speed = 84 km/h

The mean speed of all the vehicles crossing a location is known as Time mean speed during the period of study. Time mean speed is the mean of instantaneous speeds. It helps to decide the distribution of speeds of vehicles.

The 98th percentile speed is adopted for geometric design of highway.

Geometrical design speed = 98^th percentile speed
Safe speed limit = 85^th percentile speed
Minimum speed limit = 15^th percentile speed

Long Base method is a method to measure spot speed. In the Long Base method, vehicles are timed over a long distance. The second method to measure spot speed is Short Base method. In the Short Base method, vehicles are timed over a short distance of about 2m.

Radar speed meters is not a procedure of Long base method. Direct Timing procedure, Enoscope, and Pressure contact tubes are the procedure of Long base method.

The advantage of Enoscope is that it eliminates Parallax error. Enoscope is also inexpensive, simple, and easily installable. Enoscope is also called Mirror box. It is an L- shaped box with a mirror set at 45 degrees to the arms of the instrument.

The average difference between observed speeds and time mean speed is Standard deviation. 85^th percentile speed is the speed beneath which 85% of the vehicles are driven. Median is the speed that equally divides the spot speed distribution.