Sight distances

Driver Eye Height

 Sight distance is a safety design factor which is intrinsically linked to rate of change of grade , and hence to K - values 
Consider once again the hump - backed bridge 
Drivers approaching from each side of this particular vertical curve cannot see each other until they arrive , simultaneously almost on the crest ; by which time it may be too late to prevent an accident 
Had the curve been longer sight distance and consequently more time in which to take avoiding action 
Thus , sight distance i , e the length of road ahead that is visible to the driver is a safety factor and it is obvious that the sight distance must be greater than stopping distance in which the vehicle can be brought to rest 
Stopping distance is dependent upon : 
(1) Speed of the vehicle 
(2) Braking efficiency 
(3) Gradient 
(4) Coefficient of friction between tyre and road 
(5) Road conditions 
(6) Driver’s reaction time 
In order to cater for all the above variables , the height of the driver’ s eye above the road surface is taken as being only 1.05 m a height applicable to sports cars whose braking efficiency is usually very high 
Thus other vehicles , such as lorries with a much greater eye height would have a much longer sight distance in which to stop

 

  Sight distances on crests 

Short Sight Distance

Sight distances are difined as follows:

 (1) Stopping sight distance (SSD) 

The SSD is the sight distance required by a driver to stop a vehicle when faced with an unexpected obstruction on the carriageway 

It comprises two elements :

(a) The perception - reaction distance which is the distance travelled from the time the driver sees the obstruction to the time it is realized that the vehicle must stop and 

(b) The baking distance which is the distance travelled before the vehicle halts short of the obstruction 

The above are a function of driver age and fatigue road conditions , etc , and thus the design parameters are based on average driver behaviour in wet conditions Table 10.3 provides values for desirable and absolute minimum SSD 

Sight Distance

It has been shown that 95% driver’s eye height is 1.05 m or above ;the upper limit of 2 m represents large vehicles 

The height of the obstruction is between 0.26 m and 2.0 m 

Forward visibility should be provided in in both horizontal and vertical planes between points in the centre of the lane nearest the inside of the curve 

Full overtaking sight distance (FOSD ) 

On single carriageways , overtaking in the lane of the opposing traffic occurs 

To do so in safety requires an adequate sight distance which will permit the driver to complete the normal overtaking procedure 

The FOSD consists of four elements : 

(a) The preception /reaction distance travelled by the vehicle whilst the decision to overtake or not is made 

(b) The overtaking distance travelled by the vehicle to complete the overtaking manoeuvre 

(c) The closing distance travelled by the oncoming vehicle whilst overtaking is occuring 

(d) The safety distance required for clearance between the overtaking and oncoming vehicles at the instant the overtaking vehicle has returned to its own lane 

It has been shown that 85% of overtaking takes place in 10 seconds and Table 10.3 gives appropriate FOSD values relative to design speed 

It should be obvious from the concept of FOSD that it is used in the design of single carriageways only , where safety when overtaking is the prime consideration 

For instance , consider the design of a crest curve on a dual carriageways with a design speed of 100 km/h 

Height Of Driver's Eye

From Table 10.3 

Desirable minimum K-value = 100 

One step below desirable minimum K - value = 55 

FOSD K - value = 400 

As overtaking is not a safety hazard on a dual carriageway , FOSD is not necessary and one would use : 

L = 100 A ( desirable minimum ) 

or 

L = 55 A ( one step below desirable minimum ) 

Had the above road been a single carriageway then FOSD would be required and : 

L = 400 A 

If this resulted in too long a curve , with excessive earthworks , then it might be decided to prohibit overtaking entirely , in which case : 

L = 55 A 

would be used 

Although equations are unnecessary when using design tables , they can be developed to calculate curve lengths L for given sight distances S , as follows : 

(a) When S < L ( figure 10 . 42 ) 3

From basic equation y = CI2

N.B If the relationship of S to L is not known then both cases must be considered ; one of them will not fulfil the appropriate argument S > L or S < L and is therefore wrong