T. Mauga
224
were earlier developed by Raymond [2] and Glennon [3] and found them to
match. Mauga also developed a design chart for practitioners to use. Although
the chart was developed based on vehicles traveling at constant speed, it was lat-
er shown that the chart is also applicable for sites with variable operating speeds
[7]. Therefore, the chart by Mauga [4] [5] and earlier charts by Raymond [2] and
Glennon [3] are all one and the same and are in perfect agreement with their
parent that they mimic (
i.e.
the graphical method).
The graphical method and its analytical counterparts use curve geometry (
i.e.
radius and length) and stopping sight distance as input to directly output mini-
mum offsets. These minimum offsets are used as criteria for whether or not to
clear a roadside object of given offset. If an offset to a roadside object, which may
be referred to as an available offset, is less than the minimum offset then the ob-
ject has to be removed or the curve has to be redesigned. If the available offset is
greater than the minimum offset then the object is left alone since it does not
impact sight distance negatively. An example where available offsets are greater
than minimum offsets is on approach tangent sections just downstream of PC-S.
On the tangent sections significant lengths of sightlines near drivers are accom-
modated within lanes, making minimum offsets shorter than the available offset
to the boundary (
i.e.
outside edge) of the clear zone. The shorter minimum off-
sets are usually discarded and the boundary of the clear zone automatically be-
comes part of the provided clearance envelope.
2. The Problem
There has been recent studies that have proposed use of roadside clearance
boundaries or envelopes that are different from clearance envelopes resulting
from the graphical method and its analytical charts [2,3]. For example, Ameri
et
al
. [8] assumed that the roadside clearance envelope was a transformed Euler’s
spiral that starts at the beginning of a horizontal curve. However, Ameri
et al
.
did not check whether or not that envelope provided at least stopping sight dis-
tance at all locations. They could have used the graphical method to conduct that
check. Mauga [5] evaluated use of the Euler’s spiral as clearance envelope in the
way the graphical method is conducted and found that the Euler’s spiral would
provide sight distances that are less than stopping sight distance.
You and Easa [9] proposed innovative use of the Euler’s spiral as a roadside
clearance envelope near beginnings and ends of horizontal curves such that the
spiral provides sight distances that are at least equal to stopping sight distances.
You and Easa [9] also developed design charts for offsets to the Euler’s spiral (also
referred to as the innovative envelope in this paper). However, the offsets to the
innovative envelope were not compared with accurate minimum offsets deter-
mined with models already accepted in practice: minimum offsets derived from
Raymond’s chart [2] or minimum offsets determined with the graphical method.
3. Objectives
The main objective of this paper is to analyze suitability of innovative use of the