TRANSPORT ENGINEERING NOTES-DESIGN CAPACTY AND SIGHT DISTANCES.pdf

kenya-road-design-manual-for highway and expressway design

2. Traffic forecasting
Traffic forecasting is the process of estimating the number of vehicles or people that will use a
specific transportation facility in the future.
Future AADT = Current AADT x (1 + AAGR)
n

Where:
n = number of years.
AAGR = Average Annual Growth Rate used to develop the future traffic forecast.

Volume & Flow rate
The distinction between volume and flow rate is important. Volume is the number of
vehicles observed or predicted to pass a point during a time interval.

3. Flow rate represents the number of vehicles passing a point during a time interval less than
1h, but expressed as an equivalent hourly rate.


EXAMPLE




Given 15 minute flow of 1000, 900, 800, 850 v/15. What is the peak hour factor?
Solution:
V
15
= 1000 v/15.
V= ( 1000+900+800+850)= 3550 v/h.
PHF= 3550/(4*1000)=0.89.
PHF generally range from 0.85 to 1.00.

4. Number of lanes




ALIGNMENT
Sight distances
Sight distance is the length of roadway ahead that is visible to the roadway user. he four
following aspects are commonly discussed for motor vehicle sight distance:
1. Stopping Sight Distance
2. Meeting Sight Distance
3. Passing Sight Distance
1.Stopping Sight Distance
Stopping sight distance (SSD) refers to the minimum distance required for a driver to safely stop
their vehicle while traveling at a given speed.
SSD takes into account factors such as the driver's reaction time, the vehicle's braking
capabilities, and the grade of the road. It's typically calculated based on the following formula:

5. Example:
Calculate the stopping sight distance for a road with a design speed of 100 kilometers per hour
(km/h) on a level surface. Assume a reaction time (tr) of 2.5 seconds and a coefficient of friction
(f) of 0.7 for dry pavement conditions.
Solution:
First, we need to convert the design speed from km/h to meters per second (m/s) since the
formula requires consistent units:

6. 2. Meeting Sight Distance
Meeting sight distance (MSD) refers to the distance required for two drivers traveling in opposite
directions along a roadway to safely see each other and react to avoid a collision.


Example:
calculate the meeting sight distance (MSD) for a two-lane road with a design speed of 80
kilometers per hour (km/h). Assume a driver's eye height of 1.1 meters above the road surface, an
object height of 0.15 meters, and a perception-reaction time (tr) of 2.5 seconds.
Solution

7. 3. Passing Sight Distance
Passing sight distance (PSD) is the distance required for a driver to safely overtake or pass
another vehicle traveling in the same direction on a two-lane road without causing a collision
with an oncoming vehicle.
Example:
calculate the passing sight distance (PSD) for a two-lane road with a design speed of 80
kilometers per hour (km/h). We'll assume the overtaking vehicle can accelerate at a rate of
2 m/s22m/s2, the driver's perception-reaction time (tr) is 2.5 seconds, and the length of the
passing zone is 500 meters.