Bike turning radius and stable speed

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Specifications for the design of cycle paths include the following recommendations:

  • ‘The inside radius should be a minimum of 2.5m to allow for cycle manoeuvring without dismantling.’ (Auckland p64);
  • ‘The maximum design speed for cyclist on Local Paths in parks and open spaces is 20km/h’ (Auckland p17);
  • ‘The design speed of a cycle track should never be less than 10 km/h out of consideration for comfort as well as safety (stability)’ (Dutch Crow p23); and
  • ‘The width of the Local Path should balance the requirement for a safe environment with the practical cost of the path. In most cases a 3m unmarked shared path will be adequate.’ (Auckland p56 & 57)
Auckland Local Path Design Guide Rev 1.2 page57

What is a comfortable and stable biking speed? The answers include the following estimates:

  • ‘A human being travelling on a bicycle at 16–24 km/h, using only the power required to walk’ (Bicycle performance wiki).
  • ‘For cyclists in Copenhagen, the average cycling speed is 15.5 km/h, in places with green wave for cyclists 20.72 km/h’ (Bicycle performance wiki *10).
  • A cyclist …is at his most comfortable travelling at 16 to 20 km/h (Dutch CROW p20).
  • ‘At low speeds, cyclists are prone to wobble and deviate from a straight line. For most cyclists, a speed of 7 mph (11 km/h) or more is required to ride comfortably in a straight line without a conscious effort to maintain balance’ (UK LTN 2/08 p16)
  • ‘If cyclists are forced to cycle slower than 11 km/h, they need more clear space to be able to keep their balance by movement of the handlebars’ (CROW p16).

It was seen that the bike’s minimum stable speed is between 3.8 and 5.4m/s [13.68 and 19.44km/h] (The stability and handling characteristics of bicycles Stevens p45)

The above images come from the Auckland Local Path Design Guide 1.2 (Auckland p65). For local mixed use paths, speeds do need to be managed. ‘[At] speeds over 20km/h, curves with a radius smaller than 15m will be experienced as a tight curve, and will require a person on a bike to reduce their speed … a radius smaller than 5m will help to slow a person on a bike to a speed which will allow them to mix safely with pedestrians and other users without dismounting’ (Auckland p63).

Care needs to be taken when using less than an R2.5 inside radius. The UK Cycle Infrastructure Design ‘LTN 2/08’ suggests a very tight ‘turning circle’ which I could not use and I am sure someone new to biking on a normal bike cannot actually ride, so before using this table, designers of paths should establish that they can ride these curves themselves. This focus on the absolute minimum is unhealthy.

LTN 02/08 Cycle infrastucture design page 18

I need to repeat this: any designer who thinks the values presented in ‘Table 2.4’ are applicable in the real world should spend some time with people new to biking to see with their own eyes what is bike-friendly and safe.

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West Town Belt – Victoria St / Edgecumbe underpass

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The West Town Belt Master Plan is now up for submissions on Hamilton City Council’s “Have your Say” page. For this post I thought I would look back to the 1988 Draft ‘Town Belt Management Plan’ with regard to the Edgecumbe Park concept plan, focusing on  ‘Possible future pedestrian underpass combined with the Waitawhiriwhiri Stream pipe to link the W.T.B [West Town Belt] walkway with the river walk system’

Recent sandbagging shows the idea of building a wall and using the existing floor as a pedestrian underpass, with an access path extending under the footpath at the bridge, then looping round and connecting up to the existing river path.

The idea looks like a practical way of providing a safe crossing under Victoria Street. However, there could be a concern with the existing floor being at an elevation of about 12m above sea level, which means it will regularly close due to flooding. As a reference the Grantham Street boat ramp car park is 14m to 15m above sea level.

Option ‘B’ is to build a raised floor. The existing river path bridge is too close to ramp up to, so there is a need for a bridge realignment, or to move or even build a new second bridge.

The existing river path bridge is 2.7m wide, supported by 3 universal beams spanning 20m. Its hand rail tapers out to 3.2m at bike handlebar height[, and its centre line is 5.5m away from the Victoria Street retaining wall. With the river path (Te Awa river ride) becoming busier and busier in the near future, bridge widening may be a good idea here anyway.

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