CAN Do 2010 session 1: Safer speeds

CAN Do 2010 session 1: Safer speeds

Submitted by adrian on 10 November, 2011 - 09:44

1. Effect of speed on safety

  • Risk as a function of car impact speed- different curves. Perhaps overstated a bit in the past. Slope is the important thing.
  • NZ Crash Analysis System (CAS) data 2005-2010- needs more work but same pattern for higher speeds.
  • User factors- age, gender. Hazard perception, risk acceptance
  • Vehicle factors
  • Road/ environment factors- density, weather, road condition, lighting; speed limits, enforcement; lane widths, sight distances; traffic management/calming devices

2. Speed limits

Balance between: safety, efficiency, encouragement of active transport modes, preservation of amenity

Must be: credible, consitently applied, clear

Good practices:

  • Limits appropriate to road function
  • Adequate length
  • Regular signposting
  • Don't apply to compensate for hazards
  • Is it really the limit that needs changing?

Survivable speed limits

  • Match limits and roads, based on survivability and potential damage
  • Pedestrians/cyclists: 30 km/h
  • side-on crashes: 50 km/h
  • head-on crashes:70 km/h
  • fully protected from above dangers: 90 km/h +

Setting of speed limits rule 2003

  • All RCAs responsible for setting limits on their roads (was central government via LTSA under various acts)
  • All existing limits transferred into a new local Bylaw by 2009- recorded in Speed Limits Register (online)
  • Default urban 50 km/h, rural 100 km/h- all others specifically noted
  • Can set in multiples of 10 km/h.  And holiday/temporary/variable limits (e.g. School zones; motorway)
  • Set procedures for determining/ setting limits

NZ process for setting speed limits:

  • Consider mean etc. operating speeds, developments, characteristics etc.
  • Rating tables- development, roadway- also for cyclists, parking, traffic controls and land use
  • Existing usage only- doesn't look at potential usage
  • Then calculate speed limit- tally up points- flowchart.  But some discretion possible to set lower speeds in some cases.
  • Any scope for revisiting methodology- is conservative? ('lethal/ irresponsible'?)

Rural speed zones- limit set for operating conditions/characteristics- lower than 100 km/h (e.g. Kaikoura coast)- usually no less than 10 km long. Criteria: width, shoulders, median barriers, straight, speeds generally used. Various trial sites at present.

40 km/h school zones- Christchurch had the first (2000).  Variable- lowered to 40 km/h at start/end of school day. Need >50 kids on roadside.  Mean traffic speeds > 45 km/h, or 85% > 50 km/h, or speed-related crashes in last 5 years.   Use variable display flashing signs. Encourage some police enforcement. See NZTA Traffic Note 37 for more. Should push for some 30 km/h zones?

Can use 10-40 km/h limits when rating = 50 km/h. School. residential, shared zones (malls, carparks etc.)  Only for local/ minor collector roads.  Mean speeds within 5 km/h of limit. Little guidance on using these.

Sometimes encounter the 'our hands are tied' response- not true.  Can depart from calculated limit if certain conditions apply- but fairly vague. You need good evidence to support a different speed limit.

Consultation- still have to do it even if the setting process is done. Who is consulted?   local communities, police, AA, RTF, 'any other organisation' (are you on the list?)

3. Common responses to lower speeds

  • Longer travel times: negligible compared with intersections.   Trading some mobiliity for big gain in safety/ sustainability.
  • Already going slower than limit: so reinforce that true limit.  Also sends a clear message to walkers and cyclists.
  • Public don't want them- depends who you ask- people who live there?
  • Traffic will still go faster than limit- may have to do traffic management to get speeds down. May also save cost on e.g. cycle lanes.
  • "This is not Europe": we're all humans.  Crashes have the same effect at high speeds everywhere.
  • Problems: street length (visual/ physical), width (perceived/ actual), smooth surfacing
  • Effect of geometry on speeds- TRL report 2007. Speed vs. visibility.

4. Devices

  • Perimiter treatments-narrowing, signage, islands etc.
  • Speed treatments, surfacing.
  • Initiatives- "self explaining roads" in Auckland.  Hamilton's 30 km/h CBD zone.  Wellington's 30 km/h shopping streets. Christchurch's calmed neighbourhood clusters.

5. Where to find more help

6. Conclusions

  • Speed is still a problem
  • Setting limits is now easier
  • There are a number of tools- under-used in NZ
  • Do we fit road to limit or vice-versa?

Homework: think about suitable locations for :

  • lower rural limit
  • lower arterial limit
  • lower residential limit
  • lower CBD retail limit
  • Would changing limit work by itself?
  • Are engineering treatments be needed too?
  • Who would you contact?