20mph Research Analysis of speed outcomes in 20mph limit ... · GPS data for 12 case study areas 2nd March 2017 –RSGB Analysts Conference Jane Robinson and Richard Fernandes ...

30th June 2016 1

20mph Research – Analysis of speed outcomes in 20mph limit areas using GPS data for 12 case study areas

2nd March 2017 – RSGB Analysts Conference

Jane Robinson and Richard Fernandes (Atkins)

Atkins, AECOM, Professor Mike Maher

Structure of Presentation

• Study purpose and objectives, and overall methodology

• Existing evidence and wider trends in vehicle speeds

• Analysis of area-wide speed outcomes using GPS data

• Questions and discussion

2

Study purpose

• Atkins, Aecom and Professor Mike Maher were commissioned by

the DfT in 2014 to undertake research into 20mph signed only

speed limits.

• Purpose - To address an evidence gap regarding the effectiveness

of 20mph speed limit only schemes.

3

Study objectives

• To evaluate the effectiveness of 20mph speed limits, in a range of

settings.

• To examine drivers’ and residents’ perceptions of 20mph limits.

• To assess the relative costs/benefits to vulnerable groups e.g.

children, cyclists, the elderly.

• To evaluate the processes and factors which contribute to the level of

effectiveness of 20mph speed limit schemes.

Case studies

• 12 case study schemes; generally implemented 2012-2015

• 8 large residential area-wide schemes

• 2 small-scale schemes in self-contained residential areas

• 2 focused on city centre areas

4

5

Data sources and analysis

Stakeholder interviews:

• National stakeholders (DfT, PACTS, ACPO, ABD, etc.).

• Local stakeholders (officers, councillors, police, public health, bus operators, interest groups)

Social research (attitudes, perceptions and behaviours):

• Residents and drivers questionnaires (sample = ~3400)

• In-depth interviews with drivers (sample = ~200)

• 12 focus groups and online surveys with specific user groups

Detailed data analysis (speed, safety and other outcomes):

• GPS area-wide speed data (tomtom data)

• Local authority spot speed data, collected using inductive loops, radar devices or similar technology

• STATS-19 safety data (collisions, injuries, contributory factors)

• Other secondary data provided by local authorities

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Analysis of speed outcomes

30th June 2016

7

Small 20mph zones

• Two extensive studies undertaken by the Transport Research

Laboratory (e.g. Webster & Mackie, 1996; Webster and Layfield, 2003)

• 20mph zones can achieve substantial reductions in average speed, of

around 9-10 mph

• Small schemes (typically covering a few kms of road length), before

speed well above 20mph (typically around 25mph), implemented to

address location-specific safety issues

20mph limits

• Less evidence available (Portsmouth, Bristol, Edinburgh, national trial

programme of advisory 20mph speed limits across Scotland)

• 20mph limits deliver much smaller reductions in average speed, typically

around 1-2mph

• Large scale, lower before speeds (closer to 20mph)

Existing evidence on speed impacts

Wider speed trends

• Average vehicle speeds during the weekday morning peak on locally managed 'A' roads (mph)

8

22.0

23.0

24.0

25.0

26.0

Jul 07

Oct

07

De

c 0

7

Fe

b 0

8

Ap

r 08

Jun

08

Se

p 0

8

No

v 0

8

Jan

09

Ma

r 0

9

Ma

y 0

9

Jul 09

Oct

09

De

c 0

9

Fe

b 1

0

Ap

r 10

Jun

10

Se

p 1

0

No

v 1

0

Jan

11

Ma

r 1

1

Ma

y 1

1

Jul 11

Oct

11

De

c 1

1

Fe

b 1

2

Ap

r 12

Jun

12

Se

p 1

2

No

v 1

2

Jan

13

Ma

r 1

3

Ma

y 1

3

Jul 13

Oct

13

De

c 1

3

Fe

b 1

4

Ap

r 14

Jun

14

Se

p 1

4

Nov-1

4

Jan

-15

Ma

r-1

5

Ma

y-1

5

Jul-

15

Oct-

15

p

Dec-1

5 p

• % exceeding the speed limit on 30mph roads fell from 54% in 2011

to 52% in 2015 (Free flow vehicle speeds in Great Britain, DfT, 2015)

25.4mph

(Dec 2011)

23.5mph

(Dec 2015)

-1.9mph

What is TomTom data?

• Anonymised GPS data

• Available at segment level

• Each segment gives:

- Sample, Speed (avg/med), Journey Time (avg/med),

Distance, Every 5th Percentile Speed

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Sample

50 veh

80 veh70 veh

60 veh

100 vehSpeed

28mph

29mph40mph

30mph

42mph

Strengths and weaknesses

Strengths Weaknesses

Historically available Only GPS vehicles

(behaviour/affluence?)

Whole area, not spot locations Requires full segment traversal

High area sample Low individual segment sample

Over many days But aggregated days

Disaggregation (to

periods/segments)

10

Methodological challenges

• What is the average speed in an area?

(issues of aggregation, samples, distance)

• What is the right metric for measuring change in

speed?

(issues of slow moving vehicles)

11

Agreed methodology

• Use one year’s pre and post scheme data

– Split to peak and non-peak hours

• Flow and distance weight each segment’s data

• Use a two core metrics:

– Median speed

– Percent of vehicle distance driven compliantly

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Distance Before –

Peak

VKMs

Before –

Non-Peak

VKMs

After –

Peak

VKMs

After –

Non-Peak

VKMs

1,912km 3.2 million 8.4 million 4.1 million 10.1 million

Sample sizes

So what have we learnt?

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.

New 20mph limits in Residential areas

14

.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Pe

rce

nt o

f V

KM

s a

t o

r b

elo

w s

pe

ed

Speed (mph)

Before Speed Limit After Speed Limit

Before After

Cumulative speed distribution, overall (PROVISIONAL RESULTS)

Before 30mph

compliance

91%

After 20mph

compliance

47%

Median Before = 21.1 mph

Median After = 20.5 mph

Diff = -0.7 mph

85th Before = 28.2 mph

85th After = 27.0 mph

Diff = -1.2 mph

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Pe

rce

nt o

f V

KM

s a

t o

r b

elo

w s

pe

ed

Speed (mph)

Before Speed Limit After Speed Limit

Before - City Centre Post % (cumulative)

Before - Residential After - Residential

New 20mph limits in City Centre areas

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.

Cumulative speed distribution, overall (PROVISIONAL RESULTS)

Before 30mph

compliance

97%

After 20mph

compliance

65%

Median Before = 18.0 mph

Median After = 17.1 mph

Diff = -0.9 mph

85th Before = 25.4 mph

85th After = 23.8 mph

Diff = -1.6 mph

Other key findings (PROVISIONAL RESULTS)

• There was no clear difference between peak and non peak

findings (journey purpose)

• Roads with higher pre-scheme average speeds showed greater

reduction in speed when 20mph introduced

• Class of road is a large factor in the speed drivers choose to

travel

• Pre-existing traffic calming increases 20mph compliance to 67%

• There is no evidence of speed displacement impacts

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Other areas of research

• Perceptions and behaviour

• Transport outcomes – spot speeds, casualties, perceptions of

the quality of the environment for walking and cycling, increase

in active travel, displacement if traffic, etc.

• Wider impacts – health, noise and air quality, community

benefits, etc.

• Adverse / unintended impacts

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Questions and discussion

30th June 2016