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
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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.
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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
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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
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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)
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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)
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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)
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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
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.
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
