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
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.
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
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