[Talk-GB] Modal Filters and Low Traffic Neighbourhoods

Martin - CycleStreets list-osm-talk-gb at cyclestreets.net
Sat Jun 12 19:53:42 UTC 2021

> I think there are people who have come up with an algorithmic way of 
> finding LTNs.

We (CycleStreets) have been working on this over the last few months.

I'm pleased to say we now have a beta map of locations available 
specifically for the purpose of OSM mappers looking at it to find cases 
where OSM data is missing or incorrect. (Coverage is currently UK & 

Password: i-am-an-osm-mapper

This is not intended for general consumption at this stage - we are keen to 
see OSM data fixes in place so that the analysis picks up missing modal 
filters, and errors eliminated, before a more public map release.

Please have a good look round your area, to find cases where there exists a 
modal filter on the ground, but we are not picking it up. Please update the 
data in OSM :) Changes should be reflected in our analysis a few days 
later. I've listed at the end of this message the typical map data errors 
I've found while validating the data, which mappers should particularly 
look out for.

We know that the analysis is not yet 100% perfect, and some locations we 
know are missed for particular reasons. I have noted the current known 
shortcomings in the analysis at the end of this message.


The term 'modal filter' is not an exact term, and there are various 
potential interventions that could be covered. We have focussed on those 
ordinarily understood to create LTNs, i.e. locations that genuinely create 
filtering for cycling and walking, but which are not just general-purpose 
walking/cycling links between areas.

1) Bollards in the middle of a highway

This scenario creates a gap through which cycles may pass but not motor 
vehicles. (NB We do not include these on service roads, which tend not to 
be relevant.)

Example - Walnut Tree Way, Cambridge:

2) Gap in the middle of a highway

This scenario is similar but uses a gap. We have set a low threshold to 
avoid this picking up cycleways/walkways between streets. We also have a 
range of heuristics to avoid finding snickets between parallel roads and 
similar scenarios.

Example - Hulton Street, Salford:

3) Gate in the middle of a highway

Similar to scenario 1 and 2, but a gate is used, with a gap at the side. We 
have tried to eliminate cases where there is a gate providing limited 
access to a private estate or commercial area, even if the gate hasn't been 
marked as access=private, but this is hard to do 100% reliably. Please 
ensure that gates are properly marked as private.

Example - Adam and Eve Street, Cambridge:

4) Continuation cycleable

This is where a sideroad doesn't quite meet the main road, but a 
continuation allows people to cycle/walk through.

Example - Melville Road, meets Holyhead Road, Coventry:

5) Continuation walkable

Similar to scenario 4, but these are only walkable. (In many cases I 
suspect these are actually cycleable as there is no 'No cycling' sign, but 
bicycle-yes is missing in OSM.) I believe there are probably a lot of cases 
in OSM where the pedestrian link from the sideroad to the main road are not 
present. Please do check thoroughly in your area!

Example - Laverna Dale continuation, Dublin:

6) No entry except cycles: contraflow

This is where a 'No entry' sign has an 'Except cycles' plate added, 
resulting in motor vehicles not being allowed to enter an area. These are 
relatively new in the UK as the DfT only officially allowed such signage 
about 10 years ago. We have avoided the marker showing the contraflow being 
present at every point a sideroad joins to it - we've tried to get just the 
entrance from main roads.

Example - Malcolm Street, Cambridge:

7) No entry except cycles, using a plug

Similar to scenario 6, this is the older arrangement where a physical gap 
exists, providing an entry for cycles into an area, allowing them to bypass 
a No entry sign to the right. (We cover both the scenario of the drawn-in 
cycleway joining to the same node as the junction itself, as well as the 
scenario of it drawn connecting to the main road but as a different node 
just to the left of the junction.)

Example - Garlic Row, Cambridge:

8) Bus gate

This is where bus lane cameras are used to create a very short section of 
bus lane, through which only buses and cycles can legally pass without 
being fined. (We have tried to avoid cases where a long general bus lane 
alongside other traffic lanes contains a tiny section of Way.)

Example - Silver Street, Cambridge:


Typical issues in OSM data I have found while trying to validate the 
analysis are as follows - please do have a look round your area and check 
for OSM data fixes where you know there is a modal filter (as defined as 
one of the above types).

- Missing pedestrian/cycle links from the edge of a local street that 
almost joins a main road (see the 'continuation' scenario cases above).

- Gaps that physically include bollards but no bollards are present in OSM 
at one/both end(s).

- Gates not marked with access=private.

- Bollards that have not been marked as bicycle=yes.

- Missing contraflows.

- Uses of highway=pedestrian where bicycle=yes is not explicitly stated, 
despite it being obvious that cycling is not banned.

- Things where the geometry actually matches a modal filter, but are in 
areas which should be marked as access=private (and therefore not really 
modal filters insofar as general road users are concerned).

- It would be useful if continuation cases could have an est_width added, 
so we can in future eliminate cases that look like continuations but never 
actually had direct car links, i.e. genuinely always were just narrow 
footways. We already have a rule of thumb on this based on distance, but it 
would be useful to have this extra data.

- General connectivity issues where a sideroad has a continuation to a main 
road that has a cycleway along it, and the sideroad cycleway only connects 
to the main road cycleway and not the road itself also.


The analysis is quite complex (basically pattern-matching using some quite 
complex SQL statements) and we haven't yet zapped every bug.

- We haven't managed to eliminate cases of gates leading to islands of 
isolated roads (i.e. private housing estate or commercial estate) where 
that island network actually includes a walkway providing a second entry 
point. Please ensure such gates are marked as private so these can be 
explicitly known to be private entrances.

- Sometimes a bus lane that includes a very short section between the two 
sides of a dual-carriageway ends up being shown as a bus gate.

- In the cases where a continuation (local street almost touching main 
road) has a walkway, if the main road has a pavement drawn in as a separate 
Way, and the continuation walkway joins only to that pavement and not the 
main road itself, we are not currently detecting it. This is the problem 
that the pavement is not strongly associated in formal connectivity terms 
with the main road itself.

- We have a declumping mechanism that aims to de-duplicate cases, so that 
for instance a short gap with bollards at either end doesn't end up with 
potentially three matches (gap, bollard, bollard). This currently has a 
slight side-effect whereby completely different modal filters that happen 
to be very close physically have one of the two zapped. We plan to add more 
intelligent rules to avoid zapping combinations that logically are very 
unlikely to be the same thing. An example is Bene't Street in Cambridge 
(cycle contraflow), at the junction with King's Parade (gate barrier):

- We aren't yet dealing with the scenario of gates at the edges of parks, 
which need to be eliminated. There are genuine cases of modal filters 
within parks which obviously need to be kept.

- There are some edge-cases whereby a continuation actually has multiple 
spurs coming off it from the same node, and none get picked up, e.g. 
Riverside Cambridge where it joins Abbey Road:

- If a filter has been (pointlessly) chopped into two ways, we may not 
detect this. Example at Poplar's Road, Walthamstow:

Martin,                     **  CycleStreets - For Cyclists, By Cyclists
Developer, CycleStreets     **  https://www.cyclestreets.net/

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