Transportation Engineering

Highway Geometric Design

Sight distance, superelevation and the fundamental traffic-flow relation — the geometry of safe roads.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Stopping sight distance is the lag (reaction) distance plus the braking distance. On a horizontal curve, superelevation plus side friction counter the centrifugal force; the no-friction value balances it fully. The fundamental traffic relation links flow, density and speed.

The Core Formula Matrix

Lag distance:

d = 0.278 V t

(

V

in km/h,

t

in s).

Braking distance:

d_{b} = \frac{V ^{2}}{254 f}

.

Superelevation (no friction):

e = \frac{V ^{2}}{127 R}

.

Traffic flow:

q = k v

(flow = density × speed).

The ‘IIT Traps’

⚠
Speed-unit factors. The $0.278$ and $127$ constants embed the km/h → m/s conversion.
⚠
SSD = lag + braking, both terms; on gradients add $\pm g$ to friction.
⚠
Superelevation is dimensionless (a slope, e.g. $0.07$ ).

📚 Standard references

Highway Engineering — S.K. Khanna & C.E.G. Justo

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · NAT

A traffic stream has density

40 veh/km

and space-mean speed

60 km/h

. The flow is _____ veh/h.

Q2MEDIUM2 Marks · NAT

For a design speed of

60 km/h

and reaction time

2.5 s

, the lag distance is _____ m.

Q3HARD2 Marks · NAT

For a design speed of

80 km/h

on a curve of radius

200 m

, the superelevation to fully counteract centrifugal force (no friction) is _____.