2 - Measuring Traffic Characteristics Flashcards
Give 7 examples of quantative data on traffic characteristics the engineer must consider:
-Flow
-Speed
-Turning movements
-Journey time
-Delays
-Queue length
-Statson road accidents
Why is data on traffic flow needed?
-Establishing annual and seasonal trends
-Economic assesment of road improvement schemes
-Designs on intersections
What are the 2 ways flow count can be?
Either manual or automated
What is the maximum counting rate for manual counting?
600-800 using casual labour
Up to 2000 using highly trained staff
What are the 3 main elements of automated counting systems?
1 - Sensor
2 - Detector
3 - Recorder / counter
What is a sensor counting system?
A physical response to vehicle passengers
What is a detector counting system?
A system which translates signal to an electrical implulseW
What is a recorder/counter?
A system which accumulates and stores the counts
What are the advanatges of using automated counting systems?
Redcued labour, suitable for long term counts
What are the disadvanatages of automated counting systems?
No classification of vehicles, so requires sample anual count, subject to mechanical failure.
What is a Pneumatic tube (air hose) sensor?
Wheels cause an air pulse in the tube, which counts. Temporary / potable use only.
What is a piezo-electric pressure sensor?
Vehicle load causes an electric pulse proporitonal to the dynamic weight of the vehicle.
What is a tribo-electric sensor?
Wheel passage causes flexing/compression of co-axial cable.
What is an inductive loop sensor?
Wire embedded a few centimetres into / onto road detects vehicles. Oscillation changes with inductance.
What is a microwave vehicle inductor?
-Sensor and detector combined
-uSEFUL FOR COUNTING
Give 4 reasons why vehicle speeds may be measured?
-Speed limit determination
-Junction realignment
-Design of new junctions
What is the spot speed of a vehicle?
The speed of an individual vehicle measured instantaneuosly at a specific point
What is the running speed of a vehicle?
The average speed along a road excluding delays and stoppages.
When is spot speed required?
-When checking in speed limits
-To measure trends in vehicles
When is the journey speed required?
When measuring the adequacy of an existing road network
Assigning trips from an origin-destination survey or trip distribution model
What are the 4 main methods of measuring spot speed?
-Manual observation
-Electronsonic speedmeter
-Radar speedmeter
-Time lapse photography
Describe the manual observation of spot speed:
2 Observation at a know distance ( baseline)
Spot speed = baseline / time taken
What are the errors involved with manual observation of spot speed?
Accuracy of the stopwatch and reaxtion time of observers.
Limited to low traffic flows, around 150
What are the advantages of using an electronic speedmeter?
-Roadside detectors enable specific sampling to take place
-Results may be collected and processed remotely or done in the street.
What are the disadvantages involved with using an electronic speedmeter?
-Drive may expect enforcements
-All detectors can give problems with multi-lane flows
What are some characterists of a radar speedmeter?
-Gives a true spot speed
-High frequency radio waves reflected back from moving vehicle at a different frequency
What is time-lapse photography?
-Speed camera basically
Camera set at 90 degree to travel. Anaysis is tedious, possible to identify all vehicles and vehicle types
Has been superseded by videos and automatic image processing.,
What are the 3 main methods of measuring running and journey speeds?
1 - Stopwatch method
2- Registration number method
3 - Time lapse photography
What is the disadvantage of the registration number method?
- No knowledge of the performance of vehicles within the road section
-Slight bias towards faster moving vehicles especially when the observation period is relatively short compared with the journey time.
What is the advantage of time lapse photography for measuring journey speed?
Possible to record all vehicles during observation period.
What is delay?
The time spent moving at the ‘ not desired speed’
When is queue normally defined?
When traffic is movng less than walking pace (around 3mph)
What is the moving observer method (when measuring queue)?
The method involves inserting a moving vehicle into traffic.
Can observe:
-vehicles passing in opposite directions
-vehicles overtaking, and overtake by
-number of parked vehicles
-time passing fixed points
-time of stopping and starting at intersections
How many vehicles are used in the moving observer method?
2
Consider a road section, A-B, we have a value for q (flow form B to A), tA, tW, x (vehicles counted in opposite direction when vehicle is travelling A to B, and y (overtaking - overtaken) when test vehicle is travelling B to A.
What is the total time to travel from A to B and back? What is the flow in direction B to A? What is the average journey time from B to A?
time taken to travel to B and back = tA + tW
Flow in direction B to A q = (x+y) / (tA + tW)
Average time B to A = tW - y / q
