Hamilton: 2-Lane Bridges

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The theoretical maximum number of vehicles per day (vpd) on an ‘ideal’ 2-lane road is between 34,000 and 39,000 vpd, the variation coming from distances to intersections, by my calculations. The ‘ideal’ lane width is 3.5m, with 1.8m clearance to fixed obstacles close to the road (Transit NZ draft Highway Manual – State-highway-geometric-design-manual Section 6.2 Traffic Lanes); see Table 6.2 below.

My main focus is on Hamilton’s 2-lane bridges. Fairfield Bridge is at capacity, and I am using this as my benchmark. Victoria Bridge traffic counts peaked in 2005 and the assumption is that this is close to capacity. The old Victoria Bridge (before deck widening) at 2.85m, suggests we can push capacity using Table 6.2.

 Claudelands Bridge peaked in 2003. I suspect the Victoria/Bryce intersections have a bigger effect on traffic volumes than lane widths, which are wider than both the Fairfield Bridge and the old Victoria Bridge. Therefore, is it possible there would be no effect on traffic counts on Claudelands Bridge if its footpaths were widened by reducing the car lane widths?

Cobham Bridge has the highest vpd count I can find for a 2-lane bridge in Hamilton: if 34,000 vpd is the theoretical capacity, this bridge is near capacity.

The Boundary Rd (Whitiora) bridge (before 3-laning), the Massey St railway overpass, and the Wairere Dr (Te Rapa) rail bridge are of the same design widths and similar distances from intersections, so they theoretically have spare capacity.

Wairere Drive between Resolution and Hukanui (the bridge over the Kirikiriroa Stream) in 2016 had a traffic count of 13,500, which in 2017 increased by 70% to 22,900 vpd then back to 18,000 vpd in 2018.

Disclaimer on how to read the two tables above: Single year traffic counts can be misleading. Table 6.2 is a guide showing how people react when they drive along different road widths. Humans are complex so what I have calculated above may be inaccurate in real-life situations.

Measures from Tokyo and Toronto do not support wider lanes being more efficient for motorists. See Narrower Lanes, Safer Streets by Dewan Masud Karim in 2015 (from earlier post ‘safer urban car lane widths’)

“Contrary to common belief, the results clearly demonstrate that narrower travel lanes, particularly 3.0m lanes, carry the highest traffic volumes (18% higher compared to 3.5m lane) … there is no measurable decrease in urban street capacity when through lane widths are narrowed from 3.7m to 3.0m … Traffic delays on urban roads are principally determined by junctions, not by midblock free flow speeds” (p12)

Hamilton Urban Blog post – Safer urban car lane widths

Link to Hamilton Traffic counts 2013 to 2017

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