Design of traffic signal problem

 Design a traffic signal for the following right angled intersection

• Major street = 12 m (4 lanes)
• Minor street = 6 m (2 lanes)
• The peak hour volumes in each direction are indicated accordingly
  

Solution:  Assuming that the average pedestrian speed = 1.2 m/s

                Pedestrian clear time along the major road = 12 / 1.2 = 10 seconds

                Peak green time = 10 + 7 = 17 seconds

                Pedestrian reaction time = 17 seconds

                Therefore, minimum green time for vehicle on minor street = 17 seconds

                Pedestrian clearance time for minor street = 6 / 1.2 = 5 seconds

                Pedestrian green time for vehicle on major street approach = 12 seconds

                Critical lane volume on major street = 660 / 2 = 330 vehicles/hour/lane

                Critical lane volume on minor street = 180 / 1 = 180 vehicles/hour/lane

                Green time on major street approach = (330/180)*17 = 31.16 seconds

                                                                                                        ~ 32 seconds

                Adding initial amber and clearance amber of 2 seconds each

                Minimum cycle length = (2 + 17 + 2) + (2 + 32 + 2) = 57 seconds           Adopting the next multiple of 5 we have the maximum cycle length as 60 seconds

Add 3 seconds (2 seconds for major street approach and 1 second for minor street approach = ratio of volume of traffic on major street to volume of traffic on minor street)

The results for the signal timing (in seconds) are tabulated below:

Signal                    Initial        Green        Clearance            Red                Cycle

timing                    Amber                        Amber                                         length    

Major street                2             34                2                        22                60        

Minor street                2             18                2                        38                60

Design of traffic signals - concepts

Basic concepts and formulae regarding design of traffic signals

The conflicts arising from movements of traffic in different directions is addressed by time sharing principle. The design principles of traffic signal are:

• phase design
• cycle length design and 
• green splitting. 

In this regard, the concepts of saturation flow, capacity, and lost times are important.

Cycle: A signal cycle is one complete rotation through all of the indications provided.

Cycle length:  Cycle length is the time in seconds that it takes a signal to complete one full cycle of indications. It is denoted by C.

Interval: Thus it indicates the change from one stage to another. There are two types of intervals - change interval and clearance interval. Change interval is also called the yellow time indicates the interval between the green and red signal indications for an approach. Clearance interval is also called all red and is provided after each yellow interval indicating a period during which all signal faces show red and is used for clearing off the vehicles in the intersection.

Green interval: It is the green indication for a particular movement or set of movements and is denoted by Gi. This is the actual duration the green light of a traffic signal is turned on.

Red interval: It is the red indication for a particular movement or set of movements and is denoted by Ri. This is the actual duration the red light of a traffic signal is turned on.

Phase:  A phase is the green interval plus the change and clearance intervals that follow it. Thus, during green interval, non conflicting movements are assigned into each phase. It allows a set of movements to flow and safely halt the flow before the phase of another set of movements start.

Lost time: It indicates the time during which the intersection is not effectively utilized for any movement. For example, when the signal for an approach turns from red to green, the driver of the vehicle which is in the front of the queue, will take some time to perceive the signal (usually called as reaction time) and some time will be lost before vehicle actually moves and gains speed.

The signal design procedure involves six major steps. They are: 

• phase design
• determination of amber time and clearance time
• determination of cycle length
• apportioning of green time
• pedestrian crossing requirements and 
• performance evaluation of the design

There is no precise methodology for the design of phases. It is often guided by:

• the geometry of the intersection
• the flow pattern especially the turning movements and
• the relative magnitudes of flow. 

A trial and error procedure is often adopted. The first issue is to decide how many phases are required. It is possible to have two, three, four or even more number of phases. 

Cycle time is the time taken by a signal to complete one full cycle of iterations. i.e. one complete rotation through all signal indications. It is denoted by C.

As the signal is initiated, the time interval between two vehicles, referred as headway, crossing the curb line is noted. The first headway is the time interval between the initiation of the green signal and the instant vehicle crossing the curb line. The second headway is the time interval between the first and the second vehicle crossing the curb line.

The first headway will be relatively longer since it includes the reaction time of the driver and the time necessary to accelerate. The second headway will be comparatively lower because the second driver can overlap his/her reaction time with that of the first driver’s. After few vehicles, the headway will become constant. This constant headway which characterizes all headways beginning with the fourth or fifth vehicle, is defined as the saturation headway, and is denoted as h.

The saturation flow rate s= 3600/h

Start-up lost time (L)

Green time (T) to clear N vehicles

Effective green time is the actual time available for the vehicles to cross the intersection. It is the sum of actual green time (Gi) plus the yellow minus the applicable lost times.

The ratio of effective green time to the cycle length (gi/C) is defined as green ratio. We know that saturation flow rate is the number of vehicles that can be moved in one lane in one hour assuming the signal to be green always. Then the capacity of a lane can be computed as,

Saturation flow rate can be computed as, 

Lane capacity is 

y is the length of yellow interval in seconds

t is the reaction time of driver

V85 is the 85th percentile of speed of approaching vehicles in m/s

a is the deceleration rate of approaching vehicles in m/s2

g is the grade of approach as a decimal

The flowchart depicting the various stages in the design of a traffic signal is shown below

Road Safety Engineering - Problems

 ROAD SAFETY ENGINEERING PROBLEMS

• Accident Rate per Kilometer

        Number of accidents of all types per km of each highway and street classification.

        R = A/L

        R = total accident rate per km for one year,
        A = total number of accident occurring in one year,
        L = length of control section in km

• Death rate based on population
  
  Number of traffic fatalities per 100,000 populations.
  
  R = (B × 100000 )/ P
  
  R - death rate per 100,000 population,
  B = total number of traffic death in one year and
  P = population of area

• Death rate based on registration

        Number of traffic fatalities per 10,000 vehicles registered.

        R = (B × 10000) / M

        R - death rate per 10,000 vehicles registered,
        B - total number of traffic death in one year
        M - number of motor vehicles registered in the area 

• Accident involvement Rate

        Numbers of drivers of vehicles with certain
        Characteristics who were involved in accidents per 100 million vehicle-kms of travel.

        R = (N × 100000000)/V

        R - accident involvement per 100 million vehicle-kms of travel,
        N - total number of drivers of vehicles involved in accidents during the period of investigation
        V - vehicle-kms of travel on road section during the period of investigation

• Accident Rate based on vehicle-kms of travel

        Number of accidents per 100 million vehicle km of travel.

        R = (C × 100000000)/V

        R = accident rate per 100 million vehicle kms of travel,
        C = number of total accidents in one year and
        V = vehicle kms of travel in one year

PROBLEM

The Motor vehicle consumption in a city is 5.082 million liters, there were 3114 motor vehicle fatalities, 355,799 motor vehicle injuries, 6,721,049 motor vehicle registrations and an estimated population of 18,190,238. Kilometer of travel per liter of fuel is 12.42 km/liter. Calculate registration death rate, population death rate and accident rate per vehicle km.

Solution: 

 Approximate vehicle km of travel = Total consumption of fuel × kilometer of travel per liter of fuel                                                             =5.08 × 106 × 12.42 = 63.1 × 106 km.

Registration death rate can be obtained from

R = (B × 10, 000) /M

R is the death rate per 10,000 vehicles registered, 

B (Motor vehicle fatalities) is 3114, M 

(Motor vehicle registered) is 6.72 × 106.

Hence

R =(3114 × 10000) /6.72 × 106 = 4.63

Population Death Rate can be obtained from the equation.

R =(B × 100, 000)/P

Here, R is the death rate per 100,000 population

B (Motor vehicle fatalities) is 3114, 

P(Estimated population) is = 18.2 × 106.

R =(3114 × 100000)/18.2 × 106 = 17.1

Accident rate per vehicle kms of travel can be obtained from the equation below as:

R =(C × 100, 000, 000) / V

Here, 

R is the accident rate per 100 million vehicle kms of travel, 

C (total accident same as vehicle fatalities) is 3114

V (vehicle kms of travel) is 63.1 × 109.

R =(3114 × 100 × 106 )/63.1 × 109 = 4.93

Safety Barriers

SAFETY BARRIERS

• Safety barriers are designed to withstand the impact of vehicles of certain weights at certain angle while traveling at the specified speed. 
• They are expected to guide the vehicle back on the road while keeping the level of damage to vehicle as well as to the barriers within acceptable limits.
• Ideally a crash barrier should present a continuous smooth face to an impacting vehicle, so that the  vehicle is redirected, without overturning, to a course that is nearly parallel to the barrier face and with a lateral deceleration, which is tolerable to the motorist. 
• To achieve these aims the vehicle must be redirected without rotation about both its horizontal or vertical axis (that is, without ‘spinning out’ or overturning), and the rate of lateral deceleration must be such as to cause the minimum risk of injury to he passengers.

Objectives of safety barriers

• Reducing the likelihood of a vehicle crossing the central reserve and reaching the opposite carriageway.
• Minimising the damage to a barrier and vehicle, following vehicle strike and also reducing the risk to the workforce and work related congestion.
• Being maintenance-free and having a life of 25 to 50 years. 

According to the IRC (6-2000) the crash barriers shall be provided at the following locations: 

• For bridges without foot paths, concrete crash barriers shall be provided at the edge of the carriageway. 
• The type design for the crash barriers may be adopted as per IRC:5
• The design loading for the barriers shall be as per IRC:6. 
• For bridges with foot paths, pedestrian railing shall be provided on the outer side of footpath
• The railings of existing bridges shall be replaced by crash barriers
• In the urban environment traffic barriers are needed on urban motorways and primary distributors, where speeds are high and dangerous. 
• Traffic barriers should be erected on both sides of roads on embankments 6m high or more and on the outer edge of the roads where the radius is 850m or less and the embankment height 3m or more. 
• It is important to provide suitable and treatment for such type of barrier in view of safety. The ends of this barrier must either be embedded into ground by tapering down or these must be embedded into the rigid parapet wall of a culvert or specially prepared rigid parapet fit the purpose of embedding

Road side rest areas

 ROAD SIDE REST AREAS

In order to promote road safety and avoid accidents due to long and incessant driving on national highways, the Ministry of Road Transport & Highways (MoRTH) has decided to develop wayside amenities including rest area for drivers and road users. The rest area facilities are proposed to be made available under Public Private Partnership (PPP) mode for which interest has been invited by the National Highways Authority of India (NHAI).

The general guidelines for establishing rest areas are listed below:

• The governing consideration for establishing road side rest areas is that it should ensure free flow of traffic on the road and ensure minimum interference by vehicles using the facility while ensuring safety of vehicles on the road
• Rest areas should have various amenities for users, e.g. places for parking, toilets, restaurants, rest rooms, kiosks for selling sundry items, bathing facilities, repair facilities, creche etc
• Location of the rest area should not interfere with future improvements of the highway
• Rest areas should be located where highway alignment and profile are favourable
• The proposed location should not interfere with placement and proper functioning of highway signs, signals, lighting or other devices that affect traffic operation 

Traffic aid posts

TRAFFIC AID POSTS

Since full-fledged traffic stations require huge investments and personnel, they cannot be established in several locations. This gave rise to small single-sized rooms that are located in various parts of the town at a distance from the main traffic station. The function of these posts is to maintain a smooth flow of traffic, issue traffic fine challans to offenders and attend to any accidents by reporting to the main traffic station,  clearing the road of debris and ensuring smooth flow of traffic.

• Traffic aid posts should be established at regular intervals on all important highways. 
• Highway patrols should be instituted on main highways and cranes should be available to remove vehicles involved in accidents or stalled.

Design of traffic signals - Problems

PROBLEMS ON THE DESIGN OF TRAFFIC SIGNALS

1.    Given:

• Cycle time at an intersection = 60 s     
• Green time for a phase  =  27 s
• Yellow time  =  4 s 
• Saturation headway = 2.4 s/vehicle
• Start-up time lost  =  2 s/phase
• Clearance lost time  =  1 s/phase

Calculate the capacity of movement per lane

2.    In a right angled intersection of two roads, one road has four lanes with a total width of 12 m. The other road has two lanes with a total width of 6.6 m. The traffic volume of two approaching roads is 900 and 743 PCU per hour. Design the signal timing as per IRC guidelines

3.    The average speed on a roadway is 80 kmph. Under stopped conditions, the average spacing between vehicles is 6.9m. Determine the maximum flow of vehicles (road capacity)

4.    Fifteen minute traffic count on cross roads A and B during peak hour are observed as 178 and 142 vehicles per lane respectively approaching the intersection in the observed direction of heavier traffic flow. If the amber times required are 3 and 2 seconds respectively for two loads based on approach speeds, design the signal timings by trial cycle method assuming an average headway time of 2.5 seconds during green phase.

5.    The average normal flow of traffic on cross-roads A and B during design period are 400 and 250 PCU per hour. The saturation flow values on these roads are 1250 and 1000 PCU respectively. The all-red time required for pedestrian crossing is 12 seconds. Design the traffic signal by webster's method