High way and Transportation I(DIS)
ECIV 4333

Instructor : Dr. Essam AlmasriAssistants : Eng.Mustafa abou foul

Braking distance

PROBLEMS

3-20

What is the distance required to stop an average passenger car when brakes are appliedon a 2% downgrade if that vehicle was originally travelin g at 40 mi /h (40 km/h)?

Sight distance

What is sight distance?

Sight distance is the length of highway visible to a driver. A safe sight distance is the distance needed by a driver on an arterial, or a driver exiting a driveway or street, to verify that the road is clear and avoid conflicts with other vehicles. Sight lines must be kept free of objects which might interfere with the ability of drivers to see other vehicles. Features such as hills, curves in the road, vegetation, other landscaping, signs, and buildings can reduce sight distance.

Sight distance

Stopping sight distance

( )

Applicable on all highways

Passing sight distance

( )

Applicable only on two-lane highways

Stopping Sight Distance

Stopping sight distance is defined as the distance needed for drivers to see an object on the roadway ahead and bring their vehicles to safe stop before colliding with the object. .

Stopping Sight Distance

NOTE:

The sight distance at every point along a roadway should be at least that needed for a below-average driver or vehicle to stop. Stopping sight distance is influenced by both vertical and horizontal alignment. For vertical sight distance, this includes stopping sight distance or passing sight distance at crest vertical curves and headlight sight distance at sag vertical curves .

Stopping Sight Distance
Sag vertical curves

Sag vertical curves provide greater stopping sight distance during daylight conditions, but very short sag vertical curves will limit the effective distance of the vehicles headlights at night. If lighting is provided at sag vertical curves, a design to the driver comfort criteria may be adequate. The length of sag vertical curves to satisfy the comfort criteria over the typical design speed range results in minimum curve lengths of about half those based on headlight criteria.

Stopping Sight Distance

Stopping sight distance is the sum

1-The brake reaction distance ( ).

(2.5).

2-Breaking distance ( ).

.

Stopping Sight Distance

Stopping sight distance is the sum of the brake reaction distance and the braking distance .Actual stopping distances are also affected by road conditions, the mass of the car, the incline of the road, and numerous other factors .

Stopping Sight Distance

SSD for ordinary conditions

Variation for Trucks

The recommended stopping sight distances are based on passenger car.The larger and heavier truck, need longer stopping distances from a given speed than passenger vehicles.

BUT

The truck driver is able to see substantially farther beyond vertical sight obstructions because of the higher position of the seat in the vehicle.

Passing Sight Distance

Most roads and many streets are two-lane, two-way highways on which vehicles frequently overtake slower moving vehicles. Passing maneuvers in which faster vehicle move ahead of slower vehicles must be accomplished on lanes regularly used by opposing traffic. If passing is to be accomplished safely , the passing driver should be able to see a sufficient distance ahead, clear of traffic, to complete the passing maneuver without cutting off the passed vehicle before meeting an opposing vehicle that appears during the maneuver. When appropriate, the driver can return to the right lane without completing the pass if he or she sees opposing traffic is too close when the maneuver is only partially completed.

Passing Sight Distance

: .

Passing Sight Distance

When computing minimum passing sight distances on two-lane highways for design use, certain assumptions for traffic behavior are necessary, some of which offer a wide choice. The assumed control for driver behavior should be that practiced by a high percentage of drivers, rather than the average driver.

Assumptions

The overtaken vehicle travels at uniform speed. The passing vehicle has reduced speed and trails the overtaken vehicle as it enters a passing section. When the passing section is reached, the passing driver needs a short period of time to perceive the clear passing section and to react to start his or her maneuver. Passing is accomplished under what may be termed a delayed start and a hurried return in the face of opposing traffic. The passing vehicle accelerates during the maneuver, and its average speed during the occupancy of the left lane is 16 km/h [10 mph] higher than that of the overtaken vehicle. When the passing vehicle returns to its lane, there is a suitable clearance length between it and an oncoming vehicle in the other lane.

Passing Sight Distance

The minimum passing sight distance for two-lane highways is determined as the sum of the four distances.

Passing Sight Distance

d1 - Distance traversed during perception and reaction time and during the initial acceleration to the point of encroachment on the left lane. d2 - Distance traveled while the passing vehicle occupies the left lane. d3 - Distance between the passing vehicle at the end of its maneuver and the opposing vehicle. d4 - Distance traversed by the opposing vehicle for two-thirds of the time the passing vehicle occupies the left lane, or 2/3 of d2 above.

Passing Sight Distance

Passing Sight Distance

NOTES:

For Divided roads, The passing sight distance is not available.

It is not necessary to consider passing sight distance on highways or streets that have two or more traffic lanes in each direction of travel.

For roads that have special lane for overtaking, The passing sight distance is not available.

When passing sight distance is not available it equals SSD.

Criteria for Measuring Sight Distance

Height of Drivers EyeHeight of Object