Page 353 - Civil Engineering Formulas

284 CHAPTER ELEVEN

40'
19' 17'
7'
22'
Parallel parking Right-angle parking

36'
29'
17' 18'

45°-angle parking 60°-angle parking
FIGURE 11.10 Street space and maneuvering space used for various parking positions. USCS
(SI) equivalent units in ft (m): 7 (2.13), 17 (5.18), 18 (5.49), 19 (5.79), 22 (6.7), 29 (8.84),
36 (10.97), 40 (12.19).

TRANSITION (SPIRAL) CURVES

On starting around a horizontal circular curve, a vehicle and its contents are
immediately subjected to centrifugal forces. The faster the vehicle enters the
circle and the sharper the curvature is, the greater the influence on vehicles and
drivers of the change from tangent to curve. When transition curves are not pro-
vided, drivers tend to create their own transition curves by moving laterally within
their travel lane and sometimes the adjoining lane, a hazardous maneuver.
The minimum length L, ft (m), of a spiral may be computed from
3.15V 3
L (11.24)
RC
where V vehicle velocity, mi/h (km/h)
R radius, ft (m), of the circular curve to which the spiral is joined
C rate of increase of radial acceleration
An empirical value indicative of the comfort and safety involved, C values
often used for highways range from 1 to 3. (For railroads, C is often taken as
unity 1.) Another, more practical, method for calculating the minimum length
of spiral required for use with circular curves is to base it on the required length
for superelevation runoff.