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How Traffic engineers plays with speed? Definition, Types, Relations

In transportation engineering – speed is defined as the distance(s) that is covered by a vehicle in specific time (t). Speed is the most critical aspect of the geometric design – the roads are designed according to the speed and the cost of any highway project is dependent on the speed.

Traffic speed – traffic speed characteristics – how we can define speed, distance covered by an object in unit time
Speed and its role in traffic engineering

U = s/t
In transportation engineering – speed is defined as the distance(s) that is covered by a vehicle in specific time (t). Speed is the most critical aspect of the geometric design – the roads are designed according to the speed and the cost of any highway project is dependent on the speed. The greater the speed you are providing for the riders the greater will be the cost of that project.
Important Information
The speed limit on the motarway in South Asia is usually in 120 kmph – kilo meters per hour
Speed is not consistent. The speed of any vehicle depends on many factors like the location of the vehicle, the design of the roadway, the purpose for which the person is driving the vehicle, the time in which the person is riding, the congestion condition in that locality, the weather condition and the visibility on the road etc. Because all these factors are varying and are very complex to tackle, the speed is always changing even in very small amount of time.

Types of Speed

1)    Running Speed

It is that average speed of the vehicle in which the vehicle hasn’t stopped i.e. the speed in one continuous trip. The vehicle may have slowed down but haven’t stopped. The speed which is marked on the boards present on the road is called posted speed. The speed for which the road is designed is called design speed. The speed on which mostly the cars are running on the road is called operating speed.
 Important Information
The distance between Rawalpindi and Lahore if we go from Motarway is 400 km, but if we go from G.T Road it is approx. 300 km. But more time is needed in case of G.T. Road.

2)    Journey Speed or Travelling Speed

The total average speed of the vehicle calculated by dividing the total distance between two stations with the total time taken by the vehicle plus the time spent during any stoppage. Thus it includes the speed of continuous trips plus the time of any number of stoppage.
Speed is very important aspect as far as the supply chain management is concerned, like if you have to supply a particular fresh fruit which will expire after certain hours you have to consider all the critical aspects which effects the speed, similarly in traffic engineering all the speeds are considered and studied.

3)     Instantaneous – Time mean or spot speed

It is the speed determined or recorded at some specific time. It can be calculated for example by radar checking system. It is used to study the speed patterns of the traffic. It is also used to check the percentile speed. Percentile speed is used to calculate the operating speed of the roadway.
Important Information
What is the meaning of percentile? Percentile basically compares a result with certain group of people or objects rather than comparing with absolute value. For example in statics class if your score is like only 6 % students have got better marks than you and 94% students have marks less than you, then your result is 94 percentile.

How to calculate the percentile speed?

First the speed of 100 specimen vehicles is recorded and is listed in ascending order. 85th percentile of the governing speed is then calculated and the resultant will be taken as the operating speed of the highway.
85th percentile here would mean that only 15 cars out of 100 cars have speed better then the specific speed, and 85 cars out of 100 cars have speed less than that specific speed.
In 85 percentile you have to note such a speed such that only 15 % cars have speed more than this and 85% cars have speed less than this specific speed. Thus this specific speed is taken or considered as the operating speed of the roadway.

4)    Space Mean Speed

It is the average speed of the vehicles determined at specific length of the highway section. It can be determined by first marking two sections on the roadway and then when a particular vehicle crosses that section stop watch is set on and when the vehicle left the section the stop watch is set to off and then speed which is calculated will be the space mean speed.

5)     Design speed

The speed for which the highway is designed considering all the design conditions; like vertical and horizontal alignment, lane width, median, shoulders, weather condition, visibility, volume of traffic, type of traffic, no of lanes etc.
Important Information
Standard lane width in case of Motorway and most highways is 3.65 m except the local access roads in the rural areas or the dual carriage way in case of the rural areas.

Factors Influencing the Speeds

There are two basic characteristics of traffic stream which influences the speed, by traffic stream we mean combination of different types of traffic present on the highway at specific time. Like the traffic stream we see when a traffic light signal is just turned to green, we saw motorbikes, cycles, trucks, cars etc. If we are in China we would see bicycles mostly if we are in some industrial area we would see trucks mostly, in Pakistan in cities we would rather see small cars mostly.
(a)    Macroscopic characteristics of traffic
(b)    Microscopic characteristics of traffic

Macroscopic Characteristics of Traffic

By macroscopic we mean the accumulated effect of vehicular characteristics and not considering the individual effect of traffic streams i.e. the overall effect. There are 3 basic parameters
1)    Speed – U
2)    Volume or flow – q
3)    Density or concentration - K 

Speed, we have already discussed it, what is volume, it is not the multiplication of length width and depth,
Volume or flow: no. of vehicles passing through specific section of the highway at the specific time.
Flow – if time is in hours then flow is no. of vehicles per hour. Thus its unit is veh. / hr
q = veh / hr
Density: no. of vehicles occupying specific length of a highway at specific time is known as density.
The basic difference between volume and density is that volume is in terms of time (hours) while density is in terms of length of the highway section (miles)
K = veh. / mile


-    Speed (U) = ft/sec.
-    Volume (q) = veh. / hr.
-    Density (k) = veh. / mile

Macroscopic Characteristics of Traffic

 Those characteristics which takes the individual effect of the vehicles they are
(a)    Spacing (s) and headway (h)
(b)    Clearance (c) and gap (g)

(a)    Spacing (s)

It is the front to front distance or the distance from front bumper of first vehicle to front bumper of the other vehicle. Units is in ft/vehicle
1/S = vehicle / ft
K = vehicles / mile = 5280 / s
K = 5280 / s
Where k is concentration or density – veh. / mile
And s is spacing which is in ft/ veh.
Important Information
1 mile = 5280 ft.
You can measure spacing between vehicles, by taking any two vehicles travelling on the road by satellite image and then calculating the spacing from which you can calculate the density or concentration (k).

(a)    Headway (h)

It is the time a car is lacking behind another car a head of it.
Important Information
In formula f1 racing cars the score of all the vehicles is calculated by headway the time It is behind or a head of any other vehicle.
h = sec / veh.
1/h = veh. / sec.
= 3600 / h
Speed (U) – ft/ sec ; density (k) – veh / mile ; flow or vol (q)  – veh / hr.
                                    Spacing (s) – ft / veh; headway(h) – sec / veh.
q/k =u
q = u.k
-    Where u is speed in miles / hour
-    q is flow or volume in terms of veh. / hour
-     k is concentration or density in terms of veh. / mile 

  Speed Density Relation

 Flow-density relation


Speed Flow Curve 


Problem: Find the speed of a vehicle if its headway is 2.5 sec / veh. And spacing is 200 ft/ veh.
 h=2.5 sec. / veh.
s=200 ft/veh.
q=uk => u = q/k
q=3600/h = 3600 / 2.5 = 1440 veh. / hr
k = 5280 /s = 5280 / 200 = 26.4 veh. / mile
u = q/k = 1440 / 26.4 = 54.4 miles per hour - answer

Speed is the first criterion in the feasibility report – speed directly effects the cost of a highway project. Cost vs benefit ration which is calculated for any project should be in positive if that project is to be initiated – the social and economical benefits of any project is estimated.
There are 3 components of cost of any project
(i)    Construction cost
(ii)    Vehicle maintenance cost
(iii)    Time delay cost
The last two costs comprises of 70% of the total cost, while the first one comprises of 30 % of the total cost. These costs are estimated considering 40 years.
Then Benefits of that project on the plain area, rolling terrain, or hilly or mountainous terrain is considered.
Freeway: A type of highway in which minimum posted speed is 120 Kmph and level of service is A1
Expressway: a type of highway in which speed may vary a bit but level of service is A1
Then we have collectors, etc.
A policy on Geometric Design of Highway – AASHTO Green Book

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How Traffic engineers plays with speed? Definition, Types, Relations

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