284 CHAPTER ELEVEN
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
(11.24)
whereVvehicle velocity, mi/h (km/h)
Rradius, ft (m), of the circular curve to which the spiral is joined
Crate of increase of radial acceleration
An empirical value indicative of the comfort and safety involved, Cvalues
often used for highways range from 1 to 3. (For railroads, Cis 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.
L
3.15V^3
RC
7' 22'
19'17'
40'
Parallel parking Right-angle parking17'
29'
18'
36'
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).