Category Archives: News

Cobham Bridge – Wider paths

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Adding foot path width to a steel bridge looks easy, below is an example from New York.

First a bit about safety – Every second month a driver of a motor vehicle makes a mistake at the 35 km/h corner on the western end of the bridge.

Clearly the speed difference between the present 80 km/h legal speed limit and the design speed are too far apart, NZTA knows safer speeds improve safety and in the future the legal speed limit along Cobham Drive will be 60 km/h.

Transport Modelling Report – Southern Links 22/11/2013  AECOM Job No.: 60164546 / 3.6.3 – page 41.

For now Cobham Bridge is one of the heaviest trafficked 2 lane section of road in Hamilton, its 2017 traffic count of 31,100 vpd was up from 30,700 vpd in 2016. Fairfield Bridge show there is a maximum capacity a road can carry. Cobham Bridge has not yet reached its growth limit. Let’s say the ‘ideal’ vehicle lane width is 3.5m with 1.8m clearance, with a theoretical capacity of 34,000 vpd. If the volume capacity was reduced by 70% of the ‘ideal’ by changing the lanes widths to 2.8m for motor vehicles and 1.8m for cycle lanes, the maximum vehicle capacity could be about 23,000 vpd which is roughly a third less than the 2017 count of 31,100 vpd. Is it possible in 2021 when the Hamilton bypass is open and traffic volumes drop, to change the lane widths to suit 33% less motor vehicle traffic?

Going back to the start of this post, on-road bike lanes are okay for fast and confident cyclists or E-bike riders, but for the 8- or 80-year old cyclist they are unlikely to feel safe. Adding width to the existing paths looks like a sensible thing to do.

Category: News

Victoria Bridge – Lane width capacity

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First a few statements: ‘narrower cycle lanes were three to four times less safe than wider cycle lanes.’ (NZTA Report 389: Cycle Safety: Reducing the Crash Risk) and ‘A one-way cycle-track of 2.00 m [2.01m+ is good] or narrower is not a good cycling-facility’ (The Netherlands – CROW Design manual for a cycle-friendly infrastructure 1996 (Table 4.3*)

For car lanes ‘The ideal lane width is stated as 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) and “Low volume trucks (less than 5%) experience no operational problems for narrower lane widths … For buses, it suggests using 3.3m for mixed traffic conditions and 3.0m where buffered bicycles lanes exist” (Safer urban car lanes widths)

The total road width on Victoria Bridge is 9 metres. Presently each car lane is about 3.3m wide and the shoulder lanes of 1.2m maximum (reducing to not much). We could have 2.5m car lanes with 2.01m bike lanes – ‘New Zealand legislation states’ that a “lane for the use of vehicular traffic … is at least 2.5 m wide”  This would be safe for people biking, but is it reasonable and practical?

The table below starts with the ‘ideal’ vehicle lane width as 3.5m with 1.8m clearance, if the theoretical capacity is between 30,000 to 34,000 vpd. With the car lane capacity at 85% of the ‘ideal’ for Victoria Bridge – 3.3m with 1.2m shoulder – the vpd total equates to 25,000 and 29,000; which is what the above graph shows. If lane widths were changed to 77% of the ‘ideal’ – 3.0m car and 1.5m bike lanes – the count could be expected to equal about 23,000 to 26,000 vpd. The 26,000 vpd is just above what the Victoria Bridge has been carrying for the past six years (from 2012). Would this be reasonable or practical?

In theory the Victoria Bridge could be improved by changing lane widths The problem we have is that the car and bike lane widths would be at recommended minimums, and humans make mistakes, Presently at 50 km/h the person using the narrower cycle lanes has a 50/50 chance of not being killed in a crash, a 1.5m bike lane is the tipping point between safe and unsafe. So a safer speed limit would also be needed to be added to the improvements, which would have the added benefit of giving traffic a more even flow.

Summing up: would it feel safer, will more people bike instead of drive, would more people be using Victoria Bridge if single occupant car drivers did not dominate the road space, is this solution reasonable and practical?

Category: News