Transportation Engineering

Traffic Stream Parameters

Characteristics that define and describe the behavior and performance of traffic flow.

Uninterrupted Flow

Traffic flow influenced by roadway characteristics and vehicle interactions within the traffic stream.

Interrupted Flow

Traffic operations influenced by traffic control devices.

Traffic Flow (q) Formula

• q = n/t, where q is flow in vehicles per time, n is number of vehicles, and t is time interval.

• q=1/h, where h is the average time headway

• q=uk

Time Headway (h)

The time interval between the passage of front bumpers of successive vehicles.

Average Speed

Can be defined as Time Mean Speed or Space Mean Speed.

Time Mean Speed (TMS)

Average speed of all vehicles passing a point on a roadway over a specified time.

Space Mean Speed (SMS)

Average speed of all vehicles occupying a given section of roadway over a specified time.

Traffic Density (k)

• k = n/l, where k is density in vehicles per distance, n is number of vehicles, and l is length of roadway.

• k=1/s, where s is the average spacing

Macroscopic Measures

Traffic measures that reflect the overall traffic stream characteristics, such as flow, speed, and density.

Microscopic Measures

Traffic measures that reflect the behavior of individual vehicles in the traffic stream such as heading , spacing, etc.

Level of Service (LOS)

A qualitative measure describing operational conditions within a traffic stream.

Free-Flow Speed (FFS)

Mean speed of passenger cars under low to moderate flow rates.

Volume-to-Capacity Ratio (v/c)

A measure used to analyze the effectiveness of freeway segments.

Heavy Vehicle Factor (fHV)

Adjustment factor used to account for the impacts of heavy vehicles on traffic flow.

Base Conditions

Standard conditions used for traffic analysis, including minimum lane widths and traffic stream composition.

Saturation Flow Rate

The maximum flow rate of vehicles that can pass through a signalized intersection per lane when green.

Effective Green Time

The time during which vehicles are actually able to move during a signal cycle.

Discharge Headway

The time interval between the passing of vehicles at a signalized intersection.

Base Conditions for freeway

• Minimum Lane Widths: 12 ft

• Minimum right shoulder lateral clearance between edge of the travel lane and the nearest obstacle or object influencing traffic behavior of 6 ft (minimum median lateral clearance is 2ft)

• Consisting of passenger cars only

• No on-or-off ramps within 3 mi upstream or 3 mi downstream of segment midpoint

• Level terrain with no grades greater than 2%

• Driver population are familiar users

Base Conditions for multilane highway

• Minimum lane widths: 12 ft

• Minimum of 12 ft of total lateral clearance from roadside objects (right shoulder and median)

• passenger cars only

• no direct access point along the roadway

• a divided highway

• level terrain with no grades greater than 2%

• driver population familiar roadway users

• free-flow speed of 60 mi/hr or more

Three main variables (parameters) that form the underpinning of traffic analysis are:

• Flow, q

• Speed, u

• Density, k

Pre-timed operation

uses fixed timing plans regardless of traffic conditions. (small intersection)

Semi-actuated operation

activates automatically but allows manual control for minor streets. (minor streets feeding into big highway)

Full actuated operation

allows traffic signals to adjust their timing based on real-time traffic flow.(two big highways intersecting)

Components of a signal cycle

1. Clearance interval- transition from green to red for a given set of moments (a.k.a yellow)

2. Green interval

3. Red Interval

Disadvantages of traffic signals

• increasing vehicle delay

• increasing vehicle crashes

• causing a disruption to traffic progression

Advantages of traffic signals

• reduction of some types of crashes (angular crashes)

• Provisions for pedestrians to cross the street

• Provisions for side street vehicles to enter traffic stream

• Provisions for the progressive flow of traffic in a signal system corridor

Free-flow speed

Free-flow speed is the speed at which vehicles can travel without congestion, typically determined under optimal conditions.

Differences between a freeway and multilane highway

Freeways are designed for higher speeds, lack at-grade intersections, and have controlled access, while multilane highways may have varying access points and lower speed limits.

Three main performance measure for a basic freeway segment

1. Density

2. Speed

3. Volume to capacity ratio

three component parts of a freeway

1. Basic freeway segments

2. Weaving areas

3. Merge/Diverge areas

Multi lane highway characteristiccs

• speed between 40 & 60 mi/hr

• four or six lanes, often with physical median or two-way left turns

• typically located in suburban communities leading to central cities or along high-volume rural corridors

• traffic signals may be found along such highways

characteristics of freeways

• two or more lanes

• uninterrupted flow

• no signalized or stop-controlled at grade intersections

• access to and from the freeway is limited to ramp locations

• Opposing directions of flow are continuously separated

Permitted left turns

made across an opposing flow of vehicles

Protected left turn

Made without the opposing flow of vehicles. Its stops the opposing flow so you can turn.

Compound left turns

left turns are protected for a portion snd unprotected for a portion of the signal cycle

Basic three traffic stream models/relationships

1. speed vs. density

2. flow vs. density

3. speed vs. flow

speed vs. density graph

With higher speeds there is less traffic density and with lower speeds there is more traffic density.

Flow vs. Density graph

• low density vehicles can move at desired speeds so the flow increases

• At peak the space between vehicles has decrease and reached its capacity

• high density the flow starts to decrease and vehicles are moving at slow speeds

• Kj- indicates a jammed state with no movement

Speed vs. Flow graph

• At high speeds the flow increase because cars are able to drive freely

• Once the flow has hit its capacity due to cars not having much spacing the speeds will start to decrease to deal withe the congestion

Table 6.3

Freeways & Multilane highways

Table 6.4

Freeways

Peak hour factor formula

PHF= V/ (V15×4)

Table 6.5

Freeways

Table 6.6,6.7& 6.8

Use for RV’s and Trucks on Freewys

LOS Graph for Freeways

Figure 6.2

Table 6.11

• Multilane Highways

• TLC= LCr+LCl

6.12

• Multilane Highways

• Median Type

Table 6.13

Multilane Highway & Two lane highway

LOS Multilane Highway

Figure 6.4

Table 6.14

• two lane highway

Table 6.15

• Multilane Highway

Table 6.16

• two lane highways

Table 6.17

• two lane highway

Table 6.18

• two lane highways

Table 6.19

• two lane highways