Parameters of Flow | Transportation Engineering - Civil Engineering (CE) PDF Download

Traffic Studies

Traffic studies or surveys are carried out to analyze the traffic characteristics. These studies help in deciding the geometric design feature and traffic control for safe and efficient traffic movements.

Traffic Volume Study

• Traffic volume is the number of vehicles crossing a section of road per unit times at any selected period.
• Traffic volume is expressed as vehicles/day and vehicles per hour.
• Traffic volume is generally accepted as a true measure of the relative importance of roads and in deciding the priority for improvement and expansion.
• This study is used in planning, traffic operation and control of existing facilities also for planning and designing the new facilities.
• This study is used in the analysis of traffic patterns and trends.
• Classified volume study is useful in structural design of pavements, in geometric design and in computing roadway capacity. Turning movement study is used in the design of intersections, in planning signal timings. channelization and other control devices.
• Pedestrian traffic volume study is used for planning sidewalks, crosswalks, subways and pedestrian signals.
• Traffic volume counts may be done by mechanical counters or manually.
• Mechanical counters used- Pneumatic hose, magnetic detector and radar detectors.
• The main advantage of a mechanical counter is that it can work throughout the day and night for the desired period. Disadvantage of the mechanical counter-It is not possible to get the traffic volumes of various classes of traffic in the stream and the details of turning movements.

Manual Counts: The method employs a field team to record traffic volume on the  prescribed record sheets.

Presentation of Traffic Volume Data

(a) Annual average daily traffic (AADT or ADT):

This helps in deciding the relative importance of a route and in phasing the road development program

(b) Trend charts: 
Showing volume trends over a period of years are prepared.

(c) Variation charts: 
Showing hourly, daily and seasonal variations
These help in deciding the facilities and regulation needed during peak traffic periods.

(d) Traffic flow maps

(e) Volume flow diagram:
At intersections either drawn to a certain scale or indicating traffic volume are prepared. These data are needed for intersection design.

(f) Trend charts:
Showing volume trends over a period of years are prepared.

(g) Variation charts:
Showing hourly, daily and seasonal variations

These help in deciding the facilities and regulation needed during peak traffic periods.

(h) Traffic flow maps

(i) Volume flow diagram
At intersections either drawn to a certain scale or indicating traffic volume are prepared. These data are needed for intersection design.

(j) Thirtieth highest hourly volume:
Thirtieth highest hourly volume is the hourly volume that will be exceeded only 29 times in a year.

The thirtieth highest hourly volume is taken as the hourly volume for design.

Speed Studies

• Travel Time: is the reciprocal of speed and is a simple measure of how well a road network is operating.
• Spot Speed: Is the instantaneous speed of a vehicle at a specific section or location.
• Average Speed: is the average of the spot speeds of all vehicles passing a given point on the highway.
• Space Mean Speed: represents the average speed of vehicles in a certain road length at any time.
  where, V_s = space mean speed, kmph d = length of road, m. n = number of individual vehicle observations t, = observed travel time (sec) for ith vehicle to travel distance d, m.
  The average travel time of all the vehicles is obtained from the reciprocal of space mean speed.
• Time Mean Speed: represents the speed distribution of vehicles at a point on the roadway and it is the average of instantaneous speeds of observed vehicles at the sport. where, V_t = time-mean speed, kmph V_i = observed instantaneous speed of i_th vehicles, kmph n = number of vehicles observed. - V_s<V_t (Under typical speed condition on rural highway)
  Running Speed is the average speed maintained by a vehicle over a particular stretch of road, while the vehicle is in motion.
  Overall speed or travel speed is the effective speed with which a vehicle traverses a particular route between two terminals; This is obtained by dividing the total distance travelled by the total time taken including all delays and stoppage enroute.

There are two types of speed studies carried out, 

• Spot speed study 
• Speed and delay study

(1) Spot Speed Study May be useful in any of the following aspects of traffic engineering:

• to use in planning traffic control and in traffic regulations.
• to use in geometric design
• to use in accident studies
• to study the traffic capacity
• to decide the speed trends
• to compare diverse types of drivers and vehicles under specified conditions

The spot speeds are affected by physical features of the road like pavement width, curve, sight distance, gradient and road side developments. Other factors affecting sport speed are:

• environmental conditions.
• enforcement
• traffic conditions
• driver, vehicle and motive of travel.

Spot Speed is obtained by

• Enoscope
• Radar speedometer
• Graphic recorder
• Electronic meter
• Photo electric meter
• Speed meter
• photographic methods 

Do you know?

The radar speed meter method seems to be the most efficient one as it is capable of measuring the sport speeds instantaneously and also record them automatically. But this equipment is costly.

Presentation of Spot Speed Data

Average speed of vehicles:   From the spot speed data of the selected samples, frequency distribution tables are prepared by arranging the data in groups covering various speed ranges and the number of vehicles in such range. The arithmetic mean is taken as the average speed. The table gives general information of speeds maintained on section; and also regarding the speed distribution pattern.

Cumulative speed of vehicles

From this graph 85th percentile speed is that speed at or below which 85% of the vehicles are passing the point on the highway or 15 percent of the vehicles the speed at that spot. This is known as safe speed limit.

Do you know?

For the purpose of highway geometric design, the 98th percentile speed is taken. The 15th  percentile speed represents the lower speed limit if it is desired to prohibit slow moving vehicles to decrease delay and congestion.

Modal average: A frequency distribution curve of spot speeds is plotted with speed of vehicles of average values of each speed group of vehicles on the x-axis and the percentage of vehicles in that group on the Y-axis. This graph is called the speed distribution curve. This curve will have a definite peak value of travel speed across the section and this speed is denoted as model speed.

(2) Speed and Delay Study: The speed and delay studies give the running speeds, overall speeds, fluctuations in speeds and the delay between two stations of a road spaced far apart.

They also give the information such as the amount, location, duration frequency and causes of the delay in the traffic stream.

The studies are utilized in finding the travel time and in benefit cost analysis.

Fixed delay occurs primarily at intersections due to traffic signals and at level crossings.

Operational delays and caused by the interference of traffic movements such as turning vehicles, parking and un-parking vehicles, pedestrians etc., and by internal friction in the traffic stream due to high traffic volume, insufficient capacity.

Do you know?

The result of the speed and delay studies are useful in detecting the spots of congestion, the causes and in arriving at a suitable remedial measures.

There are various methods of carrying out speed and delay study, namely:

(a) Floating Car Method or Riding Check Method: A test vehicle is driven over a given course of travel at approximately the average speed of the stream.
In this method, the detailed information is obtained concerning all phases of speed and delay including location, duration and causes of delay. The average journey time

all the vehicles in a traffic stream in the direction of flow q is given by y

where, q = flow of vehicles (Volume per min) in one direction of the stream n_a = average number of vehicles counted in the direction of stream when the test vehicle travels in the opposite direction n_y = the average number of vehicles overtaking the test vehicle-the number of vehicles overtaken when the test is in the direction of q. t_ω = Average journey time, in minute when the test vehicle is traveling with the stream q. t a = Average journey time, in minute when test vehicle is running against the stream q.

(b) The license plate or vehicle number method:
The method does not give important details such as causes of declay and the duration and number of delays within the test section.

(c) Interview technique
(d) Elevated observation
(e) Photographic technique

(3) Origin and Destination Studies: The Origin and Destination (O & D) study is carried out mainly to:

• Plan the road network and other facilities for vehicular traffic and
• Plan the schedule of different modes of transportation for the trip demand of commuters. The O & D studies of vehicular traffic determines their number, their origin and destination in each zone under study.

The various applications of O & D studies may be summed up as follows:

• to judge the adequacy of existing routes and to use in planning new network of roads.
• to plan transportation system and mass transit facilities in cities including routes and schedules of operation.
• to locate expressway or major routes along the desire lines.
• to establish preferential routes for various categories of vehicle including by pass.
• to locate terminals and to plan terminal facilities.
• to locate new bridge as per trafkc demand.
• to establish desigh standards for the road, bridges and culverts along the route.

The O & D studies of vehicular traffic determines their number. Their origin and destination in each zone under study.

Do you know?

O and D studies provides the basic data for determining the desired directions of flow or desire lines.

There are a number of methods for collecting the O and D data. Some of the methods are:

(i) Road-Side Interview Method

• The data is collected quickly in short duration and the field organization is simple and the team can be trained quickly.
• The main drawback of the method is that the vehicles are stopped for interview and there is delay to the vehicular movement. Also resentment is likely from the road users.

(ii)License Plate Method

• This method is quite easy and quick as far as the field work is concerned. The method however involves a lot of office complilations in tracing the trips through a network of stations.
• This method is quite advantageous when the area under consideration is small like intersections or a small business centre.

(iii) Return Post Card Method

• The method is suitable where the traffic is heavy.

(iv) Tag on Car Method

• A pre-coded card is stuck on the vehicle as it enters the area under study. This method is useful where the traffic is heavy and moves continuously.
• The method given only information regarding the points of entry and exit and the time taken to traverse the area.

(v) Home Interview Method

• The problem of stopping vehicle and consequent difficulties are avoided altogether. Additional data including socio-economic and other details may be collected so as to be useful for forecasting traffic and transportation growth.

(vi) Work Spot Interview Method

The transportation needs of work trips can be planned by collecting the O and D data at work spots like the offices, factories, educational institutions etc. by personal interviews.

Presentation of O and D Data The data are presented in the following forms:

(i) O and D tables: O and D tables are prepared showing number of trips between different zones.

(ii) Desire line:  Are straight lines connecting the origin points with destinations.
The width of such desire lines is drawn proportional to number of trips in both directions.
The desire line density map easily enables to decide the actual desire of the road users and thus help to find the necessity of a new road link, a diversion, a by-pass or a new- bridge.
(iii) Pie charts: Diameter of circles are proportional to number of trips.
(iv) Contour Lines: The shape of the contours would indicate the general traffic need of the area.

Traffic Flow Characteristics and Studies Traffic Manoeuvres

Time Headway: 
The time interval between the passage of successive vehicles moving in the same lane and measured from head to head as they pass a point on the road in known as the time headway.

Space Headway:
The distance between successive vehicles moving in the same lane measured from head at any instance is the space headway.
Maximum flow or capacity flow is attained at this speed when the time headway is minimum.

Do you know?

The frequency of demand for lane change will be high when the speed range of vehicles in the traffic stream is high. The lane change manoeuvres and characteristics would very much depend on the number of lanes and whether it is one-way or two way movement. The merging, diverging, weaving and overtaking operations, all come under lane changes.

The number of headways per unit time is dependent on the rate of traffic flow and is therefore a direct measure of traffic volume.
With increase in speed of traffic stream, the minimum space headway increases whereas the minimum time headway first decreases and after reaching a minimum value at optimum speed on the stream increases.

Traffic Capacity Studies Traffic Volume

It is the number of vehicles moving in a specified direction on a given lane or roadway that pass a given point during specified unit of time.

It is expressed as vehicles/hour or vehicles per day.

Traffic Density:

• It is the number of vehicles occupying a unit length of lane of roadway at a given instant.
• It is expressed as vehicles/km
• Traffic volume is the product of the traffic density and traffic speed.
• q = Ku where, q = traffic volume, vehicles/hour k = traffic density, vehicle/km u = speed of vehicle, kmph.
• The highest traffic density will occur when the vehicles are practically at a stand still on a given route and in this case traffic volume will approach zero. also called jam density.

Traffic Capacity:
Traffic Capacity is the ability of a roadway to accommodate traffic volume. It is expressed as vehicles per hour per lane.

Do you know?

Volume represents an actual rate of flow and responds to variations in traffic demand, while capacity indicates a capability or maximum rate of flow with a certain level of service characteristics that can be carried by the roadway.

The capacity of roadway depends on a number of prevailing roadway and traffic conditions. Traffic volume & traffic capacity

Basic Capacity:

Basic Capacity is the maximum number of vehicles that can pass a given point on a lane or roadway during one hour under the most nearly ideal roadway and traffic conditions which can possibly be attained. Thus basic capacity is the theoretical capacity.

Two roads having the same physical features will have the same basic capacity irrespective of traffic conditions, as they are assumed to be ideal. Thus basic capacity is the theoretical capacity.

Possible Capacity:

Possible capacity is the maximum number f vehicles that can pass a given point on a lane or roadway during one hour under prevailing roadway and traffic conditions.

Do you know?

The possible capacity of a road is generally much lower than the basic capacity. In a worst case when the prevailing roadway and traffic conditions are seldom ideal. In the worst case when the prevailing traffic condition is so bad that due to traffic congestion, the traffic may come to a stand still the possible capacity of the road may approach zero. For the purpose of design, neither basic capacity nor possible capacity can be adopted as they represent two extreme cases of roadway and traffic conditions.

Practical Capacity

Practical capacity is the maximum number of vehicle that can pass a given point on a lane or roadway during on hour, without traffic density being so great as to cause unreasonable delay, hazard restriction to the drivers freedom to manoeuvre conditions.

This is also known as design capacity.

Determination of Theoretical Maximum Capacity

where, C = Basic capacity of single lane, vehicle per hours.
V = speed , kmph.
S = average centre to centre spacing of vehicles, = S_g + L_g = Minimum space gap = 0.278 Vt,(m)
L = average length of vehicle,
m_t = Reaction time, 0.7 sec (Assumed)

The space gap allowed by the driver of a followed vehicle depends on several factors such as 

• speeds of leading and following vehicles
• type and characteristics of the two vehicles
• driver characteristics of the following vehicle
• level of service
• Road geometrics
• environmental factors
• The proportion of vehicle classes in the stream.

The maximum theoretical capacity of a traffic lane may therefore be obtained if the minimum time headway H_t is known.

where C is th capacity, vehicles per hour (3600 second), and Ht is the minimum time headway in second.

Factors Affecting Practical Capacity

• Lane Width:
  As the lane width decreases, the capacity also decreases.
• Lateral Clearance:
  Restricted lateral clearance affects driving comfort, increases accident rates and reduces capacity.
• Width of Shoulders:
  Narrow shoulders reduce the effective width of traffic lanes, thus reduce capacity.
• Commercial Vehicles:
• Alignment:
  Restrictions to sight distance requirements cause reduction in capacity.
• Presence of Intersection at Grade

Do you know?

A minimum clearance of 1.85 m from the pavement edge to the obstruction is considered desirable.

Passanger Car Unit

It is common practice to consider the passenger car as the standard vehicle unit to convert the other vehicle classes and this unit is called passenger car unit or PCU.

The PCU may be considered as a measure of the relative space requirement of a vehicle class compared to that of a passenger car under a specified set of roadway, traffic and other conditions.

The PCU value of a vehicle class may be considered the ratio of the capacity of a roadway when there are passenger cars only to the capacity of the same roadway when there are passenger cars only to the capacity of the same roadway when there are vehicles of that class only.

Mathematically,

Factors Affecting PCU Values

• Vehicles characteristics
• Transverse and longitudinal gaps
• Traffic stream characteristics
• Roadway characteristics
• Regulation and control of traffic
• Environment and climatic conditions

Based on the above factors, three sets of PCU values have been worked out for.

• Urban roads, mid block sections
• Signalized intersections and
• kerb parking

The Indian Roads congress has given set of tentative PCU values or Equivalency factors factors for rural road:

Tentative Equivalency Factors Suggested by the IRC

Practical Capacity values:
The practical capacity values suggested by the IRC for the purpose of design of different types of roads in rural areas are given in table

Capacity of different types of roads in rural areas.

Parking Studies

Parking studies are useful to evaluate the facilities available Various aspects to be investigated during parking studies are:

1. Parking Demand
   • The parking demand may be evaluated by different methods. One of the methods is by making cordon counts of the selected area and recording accumulation of vehicles during the peak hours by subtracting the outgoing traffic from the traffic volume entering the cordoned area.
   • One other method is by counting the number of vehicles parked in the area under study during different periods of the day; this method is useful when the parking demand is less than the space available for parking.
   • Interview Technique-useful when parking demand is high.
2. Parking Characteristics
3. Parking Space Inventory