Elements of Design

1
Elements of Design

• This chapter discusses the elements common to all
  classes of highways and streets including
• Sight Distance
• Superelevation
• Traveled Way Widening
• Grades
• Horizontal and Vertical Alignments

2
Sight Distance

• A drivers ability to see ahead is of the utmost
  importance in the safe and efficient operation of
  a vehicle on a highway.
• Four aspects of sight distance are sight
  distances needed for stopping (all highways),
  passing sight distances (2-lane highways),
  decision sight distances (complex locations), and
  criteria for measuring these sight distances for
  use in design.
• Stopping Sight Distance 1) the distance
  traversed by the vehicle from the instant the
  driver sights an object necessitating a stop to
  the instant the brakes are applied and 2) the
  distance needed to stop the vehicle from the
  instant brake application begins.
• The height of the drivers eye is 1080 mm (3.5
  ft) and the height of the object to be seen by
  the driver is 600 mm (2.0 ft).

3
More on Stopping Sight Distance
4
More on Stopping Sight Distance
SSD 0.278Vt 0.039 V2/a Grade zero SSD
0.278Vt V2/ (254 (a/9.81)G) Grade is G
Case 3
a 3.4m/s2
-3.5
4
One-Way G 3.5
2
-3.5
One or Two-way Flat
One-way or Two way
G 0.02 or G 0.035
G 0
-3.5
4
Two-Way G 4
Case 2
Case 1
Case 4
5
Decision Sight Distance
Decision Sight Distance The distance needed for
a driver to detect an expected or otherwise
difficult-to-perceive information source or
condition in a roadway environment that may
be visually cluttered, recognize the
condition or its potential threat, select an
appropriate speed and path, and initiate and
complete the maneuver safely and
efficiently. Placement Critical Locations such
as land drops and tool plazas, where driver
errors may be experienced in digesting
complicated traffic information. The
locations
6
More on Decision Sight Distance
7
More on Decision Sight Distance
Decision Sight Distance DSD 0.278Vt 0.039
V2/a Avoidance Maneuvers A and B Where a 3.4
m/s2 DSD 0.278 Vt Avoidance Maneuvers C,
D, and E Two Examples
8
Passing Sight Distance for Two-Lane Highways
9
More on Passing Sight Distance

• 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/her maneuver.
• The passing vehicle accelerates during the
  maneuver and its average speed during the
  occupation of the left lane is 15 km/h 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

10
More on Passing Sight Distance
Exhibit 3-5 on Page 120 shows the elements of
Safe Passing Sight Distance for Design of
Two-Lane Two-Way Highways d1
0.278ti(v-mati/2) a 2.25 2.41 km/h/s,
m 15 km/h v and t see Exhibit 3-5. d2
0.278Vt2 t2 see Exhibit 3-5. d3 see
Exhibit 3-5 d4 2d2/3
11
More on Passing Sight Distance
Exhibit 3-6 on Page 124 shows the design value of
Passing Sight Distance for Design of Two-Lane
Two-Way Highways Insert the Table here
12
More on Passing Sight Distance
Effect of Grade on Passing Sight
Distance Downgrade Passed and Passing Vehicles
easy to speed up Opposite vehicle slow
down Upgrade Passed and passing vehicles slow
down Opposite vehicle speed up. Frequency and
Length of Passing Sections f(topography,
design speed, cost and/or intersection
spacing) time spent following and Average
travel speed
13
Measuring Sight Distance
Designers should check if the available sight
distance is greater than the minimum sight
distance. The available sight distance is
dependent on the height of the drivers eye above
the road surface, the specified object height
above the road surface, and the height and
lateral position of the sight obstructions within
the drivers line of sight. Height of Drivers
Eye 1080 mm (3.5 ft) for passenger cars 2330
mm (7.6 ft) for trucks Height of Object 600 mm
(2.0 ft) for SSD 1080 mm (3.5 ft) for
PSD Sight Obstructions Crest vertical curves for
tangent roadways Physical features outside the
traveled way
14
Procedures for Measuring Sight Distance
Exhibit 3-8 shows the methods for scaling sight
distance on plans Check on Horizontal
Alignments Step One At each station, identify
potential obstructions outside the traveled way
downward and upward (in two directions) and
estimate available sight distance between the
station and the ending point of the line of
sight that is controlled by every
obstruction. Step Two Compare the available
sight distance to the minimum sight
distance Question How to use computer (with a
digital straightedge) to check the Sight
Distance Requirements
15
Procedures for Measuring Sight Distance
Check on Vertical Alignments Step One Each each
vertical curve, 1) find the highest or lowest
point 2) draw a tangent line from the point
downward and upward 3) find the point where the
offset of the tangent from the vertical
curve is 1080 mm and get the station of the
point 4) find the point where the offset of the
tangent from the vertical curve is 600 mm
and get the station of the point 5) Calculate
the difference between the stations. The
difference will be the available sight
distance Step Two Compare the available sight
distance to the minimum sight
distance Question How to use computer (with a
digital transparent strip) to check the Sight
Distance Requirements
16
Horizontal Alignment

• Curve Design Controls
• The design of roadway curves should be based on
  an appropriate relationship between design speed
  and curvature and on their joint relationships
  with superelevation and side friction.
• Centripetal or lateral acceleration is balanced
  by side friction and superelevation in geometric
  design.
• Lateral Acceleration side friction
  superelevation
• or
• 0.01e f V2/127R
• Side friction varies from 0 to fmax depending on
  the speed of the vehicle
• Superelevation rate or cross slope has its limit
  or emax that is controlled by
• emax f (weather, adjacent land use, frequency
  of slow- moving vehicles, construct ability)

17
Horizontal Alignment

• A design agency normally sets up emax based on
  facility type. Caltrans has set up emax in its
  highway design manual.
• With the emax is defined and pre-selected,
  designers can choose superelevation rate e which
  is less than emax. The sum of e and side friction
  (f) balances the lateral acceleration.
• f f(V, surface, and tire) Wet surface is the
  worse case
• Several rates, rather than a single rate, of
  maximum superelevation should be recognized in
  establishing design controls for highway curves.
  A rate of 12 should not be exceeded. A rate of
  4-6 is applicable for urban design.
  Superelevation may be omitted on low-speed urban
  streets.

18
Horizontal Alignment

• Ball-bank indicator is a testing tool for
  determining comfortable f for drivers. The
  comfortable f is 0.21 for 40-50 km/h
• Electronic accelerometer is another testing tool
  used in determining advisory speeds for
  horizontal curves and ramps.
• Testing results are shown in Exhibits 3-10 and
  3-11.
• Horizontal curves should not be designed directly
  on the basis of the maximum available side
  friction factor. Rather, the maximum side
  friction factor used in design should be that
  portion of the maximum available side friction
  that can be used with comfort and safety by vast
  majority of drivers.

19
Horizontal Alignment
Distribution of Superelevation (e) and side
friction (f) There are five methods for the
distribution of e and f (see Exhibit
3-12) Application M 1 e and f to
1/R Highways with uniform speed flow such
as rural highways M2 fmax first e make
up Urban streets with speeds not uniform M3
emax first f make up Negative friction for
curves with flat radii M4 emax first f make up
A solution to M3 but still with negative on
average speed frictions problem M5 curvilinear
relation to A practical distribution for e over
the range of 1/R curvature.
20
Horizontal Alignment

• Design Considerations
• Design considerations in horizontal alignment
  involves the determination of maximum
  superelevation rates, minimum radius, and others
• The minimum radius is the limiting value of
  curvature for a given design speed and is
  determined from the maximum rate of
  superelevation and the maximum side friction
  factor selected for design

Rmin V2/(127(0.01emaxfmax)
21
Horizontal Alignment

• F value for these facilities is shown in Exhibit
  3-13. The minimum radius for each of the five
  maximum superelevation rates (4, 6, 8, 10,
  12) is shown in Exhibit 3-14 for design of Rural
  Highways, Urban Freeways, and High-Speed Urban
  Streets.
• Method 5 is recommended for use for these
  facilities. Method 5 assumes the f curve is
  shown in Exhibit 3-13 (dark solid line). The e
  value is the difference of the lateral
  acceleration rate and the f value for a certain
  speed.
• Exhibits 3-16 to 3-25 show the tables and curves
  derived from the Method 5 procedure.
• Very flat horizontal curves need no
  superelevation. Traffic entering a curve to the
  right has some superelevation provided by the
  normal cross slope. Traffic entering a flat curve
  to the left uses friction to sustain the lateral
  acceleration and counteract the negative
  superelevation due to the normal cross slope.

22
Horizontal Alignment
SE adjustment
SE needed
No SE needed
R
R
23
Horizontal Alignment

• Transition Design Controls
• Transition from a tangent to a curve or from a
  curve to a tangent has two parts superelevation
  transition (transition in the roadway cross
  slope) and alignment transition (transition
  curves incorporated in the horizontal alignments)
• Superelevation transition involves superelevation
  runoff and tangent run out.
• Alignment transition is made of a spiral or
  compound transition curve. When no spiral curve
  is used, the transition is called
  tangent-to-curve transition.

24
Horizontal Alignment
Tangent-to-curve transition
Superelevation Runoff
New Policy
e
Tangent Run out
1
125
0
2
Old Policy Superelevation Runoff Length is at
least the distance traveled in 2.0 s at the
design speed
25
Horizontal Alignment
Tangent-to-curve transition
Lr (wn1)edbw/?
Example 1 Assume a circular curve is designed on
a two-lane two-way undivided highway with
design speed of 40 km/h. The design e is 6.
Lr ? Example 2 Assume a circular curve is
design on a four-lane undivided highway with
design speed of 100km/h. The design e is
10. Lr ?
26
Horizontal Alignment
Minimum Length of Tangent Runout
Lt encLr/ed
Example 1 Assume a circular curve is designed on
a two-lane two-way undivided highway with
design speed of 40 km/h. The design e is 6.
Lt ? Example 2 Assume a circular curve is
design on a four-lane undivided highway with
design speed of 100km/h. The design e is
10. Lt ?
27
Horizontal Alignment
Distribution of Runoff on Tangent and Curve
Lr
0 100 100 0 67 33
Lr Distribution
Design Portion of runoff located
prior Speed to the curve No. of lanes
rotated 1.0 1.5 2.0-2.5 3.0-3.5 20-70
km/h 0.80 0.85 0.90 0.90 80-130 0.70 0.75 0.80 0.
85
28
Horizontal Alignment
Spiral Curve Transitions The Euler spiral, also
known as the clothoid, is used in the design of
spiral transition curves. The radius varies
from infinity at the tangent end of the spiral to
the radius of the circular arc at the end that
adjoins that circular arc. L
0.0214V3/RC Rmax see Exhibit 3-33 on Page
179 Given R, the minimum length of spiral is as
follow Lmin,s (24PminR)0.5 where Pmin
0.2 Lmin,s 0.0214V3/RC where C 1.2 m/s3
29
Horizontal Alignment
Spiral Curve Transitions Given R, the maximum
length of spiral is as follow Lmin,s
(24PmaxR)0.5 where Pmax 1.0 The desirable
length of spiral is as follows The distance
traveled in 2 s at the design speed of the
roadway. Exhibit 3-34 on Page 181 shows the
list of the desirable length at different
design speed. Length of superelevation runoff
is the minimum length of spiral. Length of
Tangent Run Out
Lt encLr/ed
30
Horizontal Alignment
Methods of Attaining Superelevation Four
methods are used to transition the pavement to a
superelevated cross section. Method 1 Revolve a
traveled way about centerline Method 2 Revolve
a traveled way about the inside-edge
profile Method 3 Revolve a traveled way about
the outside-edge profile Method 4 Revolve a
straight cross slope traveled way about
the outside-edge profile Exhibit 3-37 shows
these four methods on Page 185.
31
Horizontal Alignment
Axis of Rotation with a Median The inclusion of
a median in the cross section influences the
superelevation transition design of divided
highways, streets and parkways Case I The
whole of the traveled way including the median
is superelevated as a plane section Medians
lt 4m and e moderate Case II The median is held
in a horizontal plane and the two traveled
ways are rotated separately around the median
edges. Median 4-18 m. Case III The two
traveled ways are treated separately for
runoff with a resulting variable difference in
elevations at the median edges. Median gt 18 m
32
Horizontal Alignment
Minimum Transition Grades Criteria 1 Maintain
minimum profile grade of 0.5 through the
transition section Criteria 2 maintain minimum
edge of pavement grade of 0.2 (0.5 for
curbed streets) through the transition
section Example An uncurbed transition
section with ? 0.65 Criteria 1 any grade
but -0.5 - 0.5 Criteria 2 any grade but
0.85 - -0.45 and 0.45 0.85
33
Horizontal Alignment
Turning Roadway Design Turning Roadways consist
of interchange ramps, roadways, or intersection
curves for right turning vehicles. Turning
roadway design does not apply to minimum
edge-of-traveled-way design for turns at
intersections Turning roadways with V ? 70
km/h, compound curves OK V gt 70 km/h,
compound curves not OK When compound curves are
considered, 2 1 for the radius of the
Intersections large curve and smaller
curve 1.75 1 Interchanges The minimum
arc length for the curve is given in Exhibit 3-38
on Page 192.
34
Horizontal Alignment
Design for Low-Speed Urban Streets Method 2 is
often used for the design of horizontal curves on
low-speed urban streets. Exhibit 3-39 on Page
193 shows the design values of f that are
applicable to low-speed urban streets (solid
line) Superelevation is impractical in many
built-up areas. Very often superelevation is not
considered in urban streets design When
superelevation is considered, Exhibit 3-41 should
be used in selecting e given the minimum R or r
given a pre-selected e.
35
Horizontal Alignment
Design for Low-Speed Urban Streets Maximum
Comfortable Speed on Horizontal Curves is derived
from the following formula (see Exhibit
3-40) 0.01 e f max V2/127R Minimum
Superelevation Runoff Length (when e is used in
design) L 2.72fVd/C
36
Horizontal Alignment
Curvature of Turning Roadways and Curvature at
Intersections Minimum radius for turning speeds
is controlled by the turning speed of the
vehicle, normally 15 km/h. Exhibit 3-43 shows
the minimum radius given design speed for
intersection curves. Transitions and Compound
Curves are often considered in design of turning
roadways and urban streets. When spirals are
used for a transition section, the minimum length
of the spiral is given in Exhibit 3-45 on Page
204. Compound circular curves keep the radius
ratio to be 1.5 1.
37
Horizontal Alignment
Offtracking Offtracking is the characteristics,
common to all vehicles, although much more
related to the large design vehicles, in which
the rear wheels do not follow precisely the same
path as the front wheels when the vehicle takes a
horizontal curve or makes a turn. W Wc
Wn Wc N(UC) (N-1)Fa Z U uR
(R2-?li2)0.5 Fa R2A(2LA)0.5 R Z
0.1(V/R0.5) Example on Page 215.
38
Horizontal Alignment
Sight Distance on Horizontal Curves Stopping
Sight Distance Relationships among, R, M, and
S is shown in Exhibit 3-58 The sight line is
the line whose two ends have 1080 mm eye height
and 600 mm object height and whose midpoint is
840 mm high. Passing Sight Distance The
sigh line has its two ends with an eye height of
1080 mm, an object height of 1080 mm and a
midpoint of 1080 mm.
39
Horizontal Alignment
General Controls for Horizontal
Alignment Alignment should be as directional as
practical but should be consistent with the
topography and with preserving developed
properties and community values Rmin should be
avoided for a given design speed. Use R gt
Rmin Consistent alignment should be sought.
Sharp curves should not be introduced at the ends
of long tangent. For small deflection angles,
curves should be sufficiently long to avoid the
appearance of a kink. Sharp curvature should
be avoided on long hill fills. Compound curves
should be cautiously considered.