1

SPOT SPEED STUDIES

2

• Spot Speed

• Instantaneous speed of a vehicle at a specified location.

• Time Mean Speed

• Average of instantaneous speeds of observed vehicles at a spot over a period of time. vt = Σqivi/Σqi

• Space Mean speed• Average of the speed measurements at an instant of time over a

space. vs = Σqi/Σ(qi/vi)

SPEED

3

• Design of geometric features.

• For regulation and control of traffic operations.

• To establish safe speed limits.

• Location and size of traffic signs.

• For analyzing causes of accidents.

• To study the traffic capacity, problems of congestion on roads etc.

• To decide speed trends.

Why???..........

4

Methods available for measuring spot speed : • Radar Speedometer which automatically records instantaneous speed.• Photographic method.• Pressure Contact Tubes• Enoscope• Direct Timing Procedure

How??.....

5

• Instrument directly measures speed.• Speed of vehicles in both directions can be observed.

RADAR SPEEDO METERS

6

• Time-lapse camera photography can be used.

• Photographs are taken at fixed intervals of time(say one second per

frame) on a special camera.

• By projecting the film on a screen, the passage of any vehicle can be

traced with reference to time.

• Images by video cameras can be also used.

PHOTOGRAPHIC METHOD

7

• Pneumatic tubes are used.• At both reference points, a flexible rubber tube is fastened to the pavement

in perpendicular direction to the path of vehicle travel.• A vehicle crossing the tube compresses the air in it ,actuating an air impulse

switch.• When the air impulse is sent, it activates an electromagnetically controlled

stopwatch in the hands of observer.• When vehicle passes over second tube, stopwatch automatically stops.• Reading is noted by observer.

PRESSURE CONTACT TUBES

8

Enoscope

• Also known as mirror box

• L-shaped box, open at both ends ,with a mirror set at 45 ˚ to the

arms of the instrument.

ENOSCOPE

9

Direct timing Procedure• Simplest method• Two reference points are marked on the pavement at a

suitable distance apart and an observer starts and stops stopwatch as a vehicle crosses these two marks.

DIRECT TIMING PROCEDURE

10

• From known distance and time, speed can be calculated.

SPOT SPEED STUDIES

Average Speed of traffic (kmph)

Recommended Trap Length (m)

Below 40 2540 - 65 50Over 65 75

Trap Length:

11

• Average speed of vehicles

• From spot speed data, distribution tables are prepared by

arranging the data in groups covering various speed ranges and

number of vehicles in each range.

• Arithmetic mean is the average speed.

• Cumulative speed of vehicles

• Graph is plotted with average values of each group on X-axis and

cumulative % of vehicles travelling at different speeds.

PRESENTATION OF SPOT SPEED DATA

12

85th percentile speed: Speed at or below 85 % of vehicles are

passing the point of highway.

Only 15 % of vehicles exceed the speed at that spot.

• Drivers exceeding 85th percentile speed are considered to drive

faster than the safe speed. Hence 85th percentile speed is

adopted for Safe speed limit at that zone.

• 50th percentile speed: median speed.

• 98th percentile speed: used for geometric design.

• 15th percentile speed: represents the lower speed limit.

PRESENTATION OF SPOT SPEED DATA (contd..)

ith Percentile Speed: The spot speed below which i percent of the vehicles travel, e.g. 85th-percentile speed

13

Observations Table:

14

Frequency distribution curve

Frequency Distribution

0.00

2.00

4.00

6.00

8.00

10.00

12.00

10 15 20 25 30 35 40 45 50

Speed (mph)

Fre

qu

en

cy

(%

)

Pace 28-34 mph

Cumulative frequency distribution curve

Cumulative Frequency Curve

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

110.00

0 10 20 30 40 50 60

Speed Class (mph)

Fre

qu

en

cy

(%

)

P85 = 35.5 mph

Median Speed=P50= 31 mph

Study Locations

19

Thank You

Any queries?????