John R. Tobin, P.E., P.T.O.E.
|
October 28, 2002 |
U. S. Access Board Comments
These comments pertain to proposed rule making on pedestrian signal phase timing
(1105.3).
The proposed walking speed criteria coupled with the proposed crossing distance
criteria would nearly double the pedestrian clearance time requirements in many
instances (see enclosed calculations for six-lane/six-lane arterial
intersections). Such large increases in pedestrian crossing times would increase
the minimum signal cycle length to unreasonable levels that would often create
excessive delays, excessive queuing, and serious degradation in existing signal
coordination plans. Enclosed calculations indicate that existing signal systems
incorporating large six-lane/six-lane arterial intersections would likely need
to operate on cycle lengths three minutes in length in order to meet the
proposed criteria. The excessive queuing and poor signal coordination that would
result could also be expected to significantly increase vehicular accident
rates. It is likely that pedestrian safety would suffer as well. As noted in the
HCM 2000 (page 18-7), pedestrians’ propensity for taking “risk taking behavior”
increases as delays increase.
In view of the serious adverse impacts (including adverse safety concerns) that
the proposed changes would impose on many existing arterials, it is concluded
that the proposed changes will often be “technically infeasible” in many
existing signal systems incorporating large six-lane/six-lane arterial
intersections. Many arterials have simply not been designed and constructed with
short enough crossing distances to feasibly accommodate the changes. Other
measures such as “count-down” pedestrian signals or passive crosswalk detection
(to extend the pedestrian clearance only when needed) may be appropriate for
these existing facilities.
Accommodating the proposed changes on new six-lane arterial facilities will
require significant changes in design standards and strategies in order to avoid
excessively long pedestrian clearance requirements. Use of corner channelizing
islands and/or median refuge islands will likely be needed (see enclosed concept
drawing). These treatments break pedestrian crossings into shorter components
that can be accommodated within a reasonable system cycle length. However, these
treatments often require additional right-of-way for the corner channelization
and additional roadway width. Implementing these treatments on existing
facilities will also be expensive and disruptive in many instances.
John R. Tobin, P.E., P.T.O.E.
1555 So. Rainbow Boulevard
Las Vegas, NV 89146
Minimum Cycle Length Calculations for
Six lane/Six-lane Arterial Intersections
Assumptions: 1) Approaches include dual left-turn lanes (24’ median) and an
exclusive right-turn lane.
2) Left-turn phase demands require no more than 40% of the signal
cycle.
Current Criteria:
Xng distance = (10’ spandral) +6(12’) + (24’ median) + ½ (12’) = 112’
Mini. ped clearance = 112’ ÷ 4 fps = 28 sec.
Min. cycle length = [2(28” + 7” Walk)] ÷ 0.60 = 117 sec.
Proposed Criteria:
Xng Dist. = 2(10’ spandral) + 7(12’) + (24’ median) + 1(8’ ramp) = 136’
Min. ped. clearance = 136’ ÷ 3 fps = 46 sec.
Min. cycle length = [2(46” + 7” walk)] ÷ 0.60 = 177 sec.
Impacts: Pedestrian clearance is increased 64% from 28 sec. to 46 sec. per
street
Minimum cycle length is increased 51% from 117 sec. to 177 sec.
Intersection Channelization to Reduce
Pedestrian Signal Clearance Time