Presented by: Civil Engineering Academy

Traffic VolumePresented by: Civil Engineering Academy

Volume

Speed

Capacity

Design Vehicles

Number of trips

1. Volume parameters are as follows (CERM Ch. 73):◦ Average daily traffic (ADT) is calculated from the 24-hour flows measured

over less than one year.

◦ Average annual daily traffic (AADT) is calculated from the 24-hour flows

measured over one year.

◦ The design hourly volume (DHV) is calculated for the design year.

◦ The K-factor is found by dividing DHV by AADT.

◦ D is the directional factor which is equal to the percentage of traffic in the

dominant direction.

◦ The directional design hour volume is equal to D(DHV) = DK(AADT).

◦ (v/c)i is the volume-to-capacity ratio for LOSi (level of service), where v =

actual flow rate (in vph/lane), and c = ideal capacity at LOSi (in vph/lane).

ADT = average daily trafficAADT = average annual daily trafficDHV = design hour volume

DDHV = directional design hour volumeD = directional factor,DHV = design hour volume

v = actual flow ratec = design capacity

Vmi = the capacity in passenger cars

per hour per lane under ideal

conditions for a particular level of

service

Peak Hour Factor

Vi = the actual capacity in passenger cars per hour under non ideal conditions.N = numbers of lane

K-factor.

Directional Design Hour Volume

Volume-Capacity Ratio

Maximum Service Flow Rate

Service Flow Rate

Eq. 73.1 CERM

Eq. 73.2 CERM

Eq. 73.3 CERM

Eq. 73.4 CERM

Eq. 73.5 CERM

Eq. 73.6 CERM

Determine the peak hour factor of a certain

highway. Traffic data count for 2 hours is tabulated

Interval Volume

6:00 – 6:15 710

6:15 – 6:30 770

6:30 – 6:45 860

6:45 – 7:00 980

7:00 – 7:15 880

7:15 –7:30 810

7:30 –7:45 760

7:45 – 8:00 700

Solution Tabulate the hourly traffic volume combinations

Interval Volume

6:00 – 7:00 3320

6:15 – 7:15 3490

6:30 – 7:30 3530

6:45 – 7:45 3430

7:00 – 8:00 3150

Therefore the peak volume

for 1 hour is 3530 veh/hr

From the raw data, the peak volume for

15 min interval is around 6:45 – 7:00, 980

veh/ 15 min periodPHF =

3530 veh/hr

(4 periods/hr)(980 veh/period)

PHF = 0.9005

CESP Transportation, Prob. 5

CESP Transportation, Prob. 5 (continued)

CESP Transportation, Prob. 5 (continued)

CESP Transportation, Prob. 5 (continued)

• Requires estimating the number of trips that will result from a particular population or

occupancy.

• Can be solved using simple math theories.

Determine the number of 2 axle cars and 4 axle trucks passing an ATR detector. The

record shows that there are 1020 axles detected and 25% of it where for 4 axle trucks.

Solution: Simple algebra theorylet x = number of 2 axle vehicle

y = number of 4 axle vehicle

Numbers of axles passing through the

detector

2x + 4y = 1020 eq. 1

25% of the vehicles were truck

y

X + Y= 0.25 eq. 2

y = 0.25 x + 0.25 yx = 3y Combine to eq. 1

2(3y)+ 4y = 1020, thus y = 102

For x

2x+ 4(102) = 1020, thus x = 306

Important Formulas

Where:

S = average travel speed

Sf = free-flow speed

Do = critical density

D = Density

Dj = Jam density

v = rate of flow

Eq. 73.9 CERM

Eq. 73.10 CERM

Eq. 73.11a CERM

Eq. 73.11b CERM

Average Travel Speed

Flow (Rate Of Flow),

Spacing

Headway

Determine the capacity ,in vph, of a single lane for private cars moving at an average

speed of 60 mph and a density of 50 vpm.

Single lane

Solution:

Mean speed = 60 mph

D = 50 vpm

Capacity (flow) = speed x density C = x60 mile

hour

50 veh

1 mile

Answer: C = 3000 veh/hr

Eq. 73.10:

Determine the site flow if the mean speed is 30 mph in a road having a density of 23 vpm.

Solution:

S = 30 mphD = 23 vpm

V = SD = 30 mph x 23 vpm = 690 vph

Determine the average speed, in mph, that a passenger car should travel in certain

freeway if the spacing of cars moving in the same lane is 130 ft center to center. Given

volume of traffic is 2000 vph.

Solution:

Spacing = 130 ftV = 2000 vph

2000 veh = (3600 sec/hr) x S

130 ft

S = 72.22 ft/sec

72.22 ft

secS = x x = 49.24 mph

1 mile

5280 ft

3600 sec

Hr

• For convenience in measuring the vehicular flow, it is important to designate a

standard.

• Since passenger cars make up the majority of highway traffic, so it is natural to

use passenger cars as the unit.

For vehicles other than passenger cars, standard E – value is set by

HCM(Highway Capacity Manual 2010)

Six-Min Problems - Transportation Prob. 45, Depth

Six-Min Solutions - Transportation Prob. 45 (continued)

Six-Min Solutions - Transportation Prob. 11

There are other topics not listed on the latest

2015 specification that could creep in under

the “not exclusive or exhaustive category.”◦ Super elevation

◦ Pavement Design

◦ Acceleration and Deceleration

More practice problems!

Next topic: Materials!◦ Soil Classification

◦ Soil Properties

◦ Concrete

◦ Structural Steel

◦ Material Test Methods

◦ Compaction