Saturday, 31 May 2014

Fun With Moderate & Controlled Variations In Vertical Carriageway Alignments (aka I've Got The Hump)

Speed humps, sleeping policemen, biscuits, cushions, tables, side entries, raised crossings, thumps: all names I have heard used to describe the humble road hump.

I have a bit of a love-hate relationship with Road Humps (to use the proper and legally defined term) because if used properly, they will get traffic speeds down. They can also be used to provide level pedestrian crossing routes and priority cycle tracks. The problem with them is that they can cause noise and vibration to those living near them, they are often uncomfortable to cycle over and they are another thing to maintain out on the street.

This dropped kerb is still on a hump, but a kerb upstand has been
left for blind and partially-sighted people to be able to find the edge
of the footway. If the whole footway is flush with a hump, then we
have the potential issue of trying to put tactile paving in everywhere
which can be a mess and doesn't work round corners unless we are
aiming to guide people into the middle of the road!
The Basics
Highway authorities are enabled to build road humps by virtue of Section 90A of the Highways Act 1980 (as amended). There are several other sections (down to 90F) which set out the particulars - have a read yourself as it is a bit too boring to explain here in full, but there are definitions, the requirements for consultation and special cases in London (where something not defined elsewhere is proposed). 

Humps are technically road obstructions and so have been specifically allowed within the law (so long as they are built within the parameters of the regulations).

Section 90D essentially gives the Secretary of State the power to make Regulations controlling the use of road humps and this has spawned the Highways (Road Humps) Regulations - we are currently on the 1999 version. These Regulations deal with a variety of matters;
  • Consultation,
  • Dimensions, nature of humps and location of humps,
  • Lighting,
  • Traffic signs,
  • Relaxations within 20mph Zones (generally signage and lighting)
Consultation must include the police, an advert in the local press and site notices (S90C HA1980) and could include a local inquiry. In addition, the fire brigade, ambulance service and

"organisations appearing ... to represent persons who use the highway to which the proposal related, or to represent persons who are otherwise likely to be affected by the road hump". (S3 HRHR1999).

These other persons could include residents and businesses, cycling groups, local conservation groups and so on - although it is up to the highway authority who is consulted (it is usual to maintain a standard list which includes all of the groups required for various pieces of law to make life easier).

For dimensions/ nature, we have a lot of flexibility. They must be built at right angles to an imaginary centre line of the carriageway, be at least 900mm long (so no rubber retail park humps!), at least 25mm high, be no more than 100mm high and have no vertical face more than 6mm high. There is no actually regulation on the shape of humps or ramp gradients, but I will cover some of that later.  We are also allowed to put zebra and signalised pedestrian crossings on top of humps (subject to a little more regulation). Additionally, they should not be used close to (or in!) tunnels, bridges, level crossings and tram tracks. 

Your basic, bog standard round top road hump.
Types Of Hump
The regulations allow a great deal of variety, but there are three common types of hump in use; round top humps, flat-top humps (sometimes called speed tables) and cushions. Round top humps have a circular profile and are normally 3.7 metres long (in the direction of travel, not across the carriageway).

The 3.7m dimension is a remnant of the 1986 Regulations which prescribed the shape of the hump (which was based on prior research), but nothing stops us making them shorter (down to 900mm) or longer. A short hump for the same given height will be harder to pass over and will lead to more noise/ vibration and a longer one will be less effective. Subsequent regulations gradually relaxed what could be done.

A sinusoidal road hump lovingly crafted from 55/10 HRA which is by
far the best material to use in my opinion as it is pretty much
bullet proof!
A development of round top humps is the sinusoidal hump which has (you guessed it) a sinusoidal cross-section. It means that in the direction of travel, the hump starts quite flat and then gradually gets steep. It then curves over the top and then gradually gets less steep on the other side. 

There is more to read in Traffic Advisory Leaflets 9/98 and 10/00 (see below), but from the point of view of riding a bike, sinusoidal humps are so much more comfortable to traverse than round top and indeed flat top with steep ramps and have a similar effect on vehicle speed as round top.

Sinusoidal humps are a little bit more difficult to construct than round top humps, but a decent asphalt gang will have no problem. As with round top humps, an area around the edge of the hump will be cut out and an asphalt "core" laid in the centre. The surfacing will then be laid over the top (allowing for compaction) and rolled to the correct levels - sinusoidal humps rely on the gang spending a little more time setting things up and a bit of skill with the roller. Really, if anyone is think if putting in humps, make them sinusoidal as it will be better for cyclists.

A set of three cushions on a wide road.
Next, we have cushions. These were developed to be able to traffic calm streets which are used by buses, ambulances and fire engines (or the odd lorry). 

The idea is that these larger vehicles can straddle the cushion so the impact is not as great for cars which have narrower wheel track. 

A lot of the skill in getting them right is their position in the running lanes and dimensions - the wider the cushion, the better the speed reduction. There is a Traffic Advisory Leaflet to assist (TAL1/98 - see link below).

A closer view of some cushions.
Cushions are not something to cycle over as the ramps are often steep. One can of course cycle between them, but they force you to either hug the channel on the left hand side (often getting close to the kerb) or play chicken in the middle of the road. Wider roads will have 3 cushions (I have seen 4!) which might be a little better for cycling. 

Unless there are parking restrictions through the set of cushions, drivers (and cyclists) are often forced into clattering over them. This often leads to complaints from those living near them.

They are sometimes used in close proximity to traffic islands and pedestrian refuges which can be very effective for reducing traffic speeds, but we have the issue of creating not only a pinch point for cyclists, but giving the choice of either passing on the left next to a kerb or on the right, next to the island. Cushions remain a compromise and I am not really a fan. Of course, there does remain the fundamental issue of needing to traffic-calm a through route in the first place which is not dealing with the issue of the appropriateness of the traffic using it.

The third type of road hump is the flat top hump (or speed table). These can be stand alone as round top humps, be used within junctions (providing a handy crossing point) or within zebra/ signal-controlled pedestrian crossings. 

Flat top humps are more forgiving than round top humps, but less than sinusoidal. Of course, a sinusoidal ramp could be used and some manufacturers make precast sinusoidal ramp segments (although they are only of use on flat carriageways rather than ones with a camber).

This flat top hump (speed table) covers an entire 4-arm junction and
provides flush dropped kerbs in all directions to help all pedestrians
cross on all arms of the junction. Quite an unusual layout as it is
normal for just the side roads to get the dropped kerbs.
Flat top humps are also useful at junctions for not only helping reduce speeds (although drivers turning in and out should be reasonably expected to slow down), but to provide a more level walking surface for pedestrians (and to reinforce pedestrians already crossing having priority).

It is also possible to give cyclists absolute priority over traffic using normal give way markings to drivers. It is not often used, but means that a cycle track crossing a road becomes the "main road" at a crossroads formed with another road. This allowance is buried within the Traffic Signs Regulations & General Directions 2002 under Regulation 25(6). The language in the regulation suggests that the hump cannot be right on a junction and so would need to be set back to provide space for a give way marking to drivers on both sides of the hump.

Cycle track with priority over traffic, near a junction.
Image from Google Streetview.
This arrangement does allow drivers to turn off a main road and then stop, but as these arrangements tend to be used in quieter places, cycle traffic probably dominates occasional access by vehicles. Compare this approach with the argument that cycle tracks can have priority anyway as they can be part of the main road. This is an area which really needs tidying up because designers tend to shy away anything which seems unusual (despite being allowed for years!)

Flat top humps are more suitable for bus routes and perhaps busier roads often used by emergency vehicles. In these locations, ramp gradients of 1 in 20 or 1 in 15 might be suitable rather than the steeper 1 in 12 (seen as the maximum in line with accessibility for people using wheelchairs). The emergency services do not automatically object to humps, but they need to be engaged from the start.

There are other permutations of humps course. One might built a hump kerb to kerb with localised drainage works (so the carriageway doesn't flood!) but this can add quite a bit of cost. the hump can be stopped short of the kerb (see the sinusoidal example above). A hump stopped short of the kerb can be a hazard for cyclists (getting trapped in the gap) or pedestrians (tripping) and so as usual, sites need to be dealt with on their merits. A longitudinal road marking picking out the "useable" edge of the carriageway is useful where the hump stops short of the kerb - compare the photos above (although the sinusoidal hump scheme was still waiting for this when I took the photo).

My examples have so far been all constructed from asphalt, so here is a flat top hump constructed from block paving. 80mm thick paviours should be used to take traffic loading and a good foundation is essential. The other important point is that all edges of block paved area need good lateral restraint or they will spread under wheel load and ruts will form.

This hump has granite setts for ramps and because it was built in a 20mph Zone, the triangular hump road markings were not needed as the hump is very conspicuous. Again, care is needed when using non-asphalt materials for ramps as they take the brunt of wheel loadings and unless well-detailed, they will fail.

Current Guidance
OK, I have said before that we should be wary of "old" guidance as it might not be current. Where humps are concerned, guidance is showing its age, but much is relevant. The Traffic Advisory leaflets are a good starting place as follows;
  • TAL7/96 - Highways (Road Humps) Regulations 1996 - yes out of date, but not much changed from 1996 to 1999 other than lighting requirements (because of a standards change) and a relaxation on signing and lighting in a 20mph Zone (more on that later),
  • TAL1/98 - Speed Cushion Schemes - essentially the outcome of research into their use and suggested optimum dimensions for different situations,
  • TAL9/98 - Sinusoidal, S & H Road Humps - a guide to some strangely shaped humps designed to slow down cars, but not affect other users (emergency services and cyclists for example). I have only ever use sinusoidal from this list and to be honest, the H and S humps seem to be a colossal pain in the backside to build and maintain
  • TAL10/00 - Road Humps: Discomfort, Noise & Ground-borne Vibration - again, this document gives some research into the subject and is actually quite interesting. In terms of user discomfort, it shows that sinusoidal humps are more forgiving to cyclists. For buses and other vehicles, suspension technology has moved a lot in 14 years and so the results will be less relevant. The vibration part is interesting and shows how the soils underlying a road can have a big impact

Final Thoughts
Like any design element, road humps have their place and will no doubt still be used. They have limitations and as usual, should be used in the right place. I can think of lots of locations where humps have been used to slow traffic, where the real issue is that the traffic shouldn't be there in the first place. 

Humps can therefore be symptom of the problem and not a cure for the problem, especially if high levels of rat-running traffic still remain with the added issue of noise and vibration from the humps and the impact on pedestrians and cyclists.

*thanks to @nuttyxander for the inspiration for the title of this post ;-)


  1. Thanks for the info.

    I'm hoping increased prevalence of 20 mph zones, and their subsequent enforcement (if that ever happens), will remove the need for humps at all.

    They frustrate motorists, they cause noise for locals and cost money to instal and maintain. Worse: they don't actually stop speeding as white vans, with deeper section tyres, are completely unaffected by them and can take them at a gallop.

    But by far my biggest problem with them is how dangerous they are for cyclists. Not only are they uncomfortable to bounce over but they cause motor traffic to do erratic things. You only realise how much you rely on your hearing as a cyclist to let you know what the vehicle approaching from behind is doing; if they engage second gear for a corner you can expect a left hook but humps remove these clues because cars are continually slowing down and speeding up. Cushions are even worse causing motorists to veer all over the road in erratic fashion.

    Let's tell humps to hump off.

    1. As usual, humps are a tool, the skill is where they are used - and they can be done well (sinusoidal are fine for cycling on most bikes). The problem with 20mph Zones is that unless through traffic is removed, then heavier traffic calming is needed and humps tend to be the only option where there is lots of on-street parking. Plus, the police seem uninterested/ unresourced to enforce, so schemes need to make sure people are driving within the limit.

  2. I've got to be honest, and say I don't share your enthusiasm for asphalt as the best construction material. It's obviously the material we are most comfortable with, but, simply from travelling around, it seems the skills required to form it properly into the variety of humps and cushions are rare - particularly the round-top humps. I've managed to ground cars on this type when going at the slowest possible speed (first gear, no accelerator, engine torque only), never mind the fifteen or twenty mph you should be able to drive over them (though not necessarily in comfort).

    If I'm using cushions I tend to go with the Marshalls precast units; they do require more extensive excavation into the carriageway, but the gradients and dimensions are all spot on to the regs. The same with their sinusoidal block paving - I hope the Contractors who formed the asphalt example in your pictures get a good bonus - that's a skill that should be recognised.

    Andy R.

    1. The gang who built the sinusoidal hump scheme were old hands having built many in Camden where they are kind of standard if humps are being used. My experience has always been good with asphalt, but the key is to kit on the contractor to make sure you get what you want and having decent standard details (against which a contract has been priced) helps.

      The preformed stuff can be very good (but some manufacturers less good than others). The problem I find with pre-cast (especially ramp blocks) is they can be a sod with a cambered road. Pre-cast cushions can be excellent, but yes, need to be installed on a proper foundation.

      Link to Marshalls:

  3. Main roads get cycle tracks on them, minor side roads get speed tables at junctions, no markings, so give way to traffic from the right, advisory cycle lanes at least 1.5 metres wide and no centre line on busier minor side roads, many locations of filtered permeability to remove any and all routes through the neighbourhood that will allow you to go in one end and come out the other in a different location, and a 20 mph zone. Parking is in signed bays only, and wherever vehicles may not be parked in, a bulb out shall be built to extend the pavement so as to cover where the parking would be, and no pavement parking. Bricks will be used as the pavement style, except through cycle shortcuts, where asphalt can be used.

  4. The effectiveness of Speed humps depends heavily on some critical dimensions: height and length of the humps, spacing, how many there are. It is disappointing that the evaluation doesn't provide any measurements of these critical dimensions. Also,success is evaluated by speed, or how often cars travel faster than the intended speed; this should have been observed and reported more carefully.