Cul-de-sac Design

[bmerc]

Our city regulation require the lenght of a cul-de-sac to be no more then 1200 ft., this I find is a standard in the country. Many residents prefer to live on cul-de-sacs so this creates a conflict, a balance that is tough to manage!!! I would have thought there was a sound engineering or technical public safety reason but I have been unable to find it??? Some of the thing I thought may be the reason were lenght of fire hose carried by an avg. fire company, water pressure of a hydrant vs. lenght to a house, etc... All these and others I researched have too many other variables and really could not be the reason for this 1200 ft max. To better explain it to my residents, does anyone have any input, sources to research or the answer to this requirement????

 

[DJF]

"Residential Streets", 3rd Edition, is a joint publication of Urban Land Institute, Nat'l Assoc. of Home Builders, ASCE and ITE. It recommends dead-end street length controlled by traffic volume, and therefore housing density. Max. vehicle trips per day should be about 200. Using 8 to 10 trips per day per single-family home would dictate 20 to 25 homes per cul-de-sac street. Check the lot requirements in the local Zoning Ordinance to translate lot frontage into # of lots, and street length.

 

[ACtrafficengr]

A recent ITE Journal had an article about a study the City of San Antonio did, which found that when a street was longer than that without a sharp curve, stop sign or signal, traffic speeds became unacceptable for residential streets.

 

[IRstuff]

Just from personal experience, I'd agree. We live on a cul-de-sac street with about 1000 ft of straight run. And some of the residents do tend to go pretty fast down the street, although that's partly fueled by complacency of being in a cul-de-sac in the first place. The speeds at which cars come into the end loop can be quite scary, particularly if there are young children about.

Given the available power of the current generation of cars, 1500 ft might actually be too long. TTFN

 

[trafficPro]

In NJ we are governed by a statewide "Residential Site Improvements Standards". They dont list a limit on cul length directly.They limit traffic volume to 200 trips per day, That limits the length to about 25 homes. The length of the cul is thus dependent on the frontage requirement.

Not in the RSIS, but governed by other code is a requirement that a school bus may not back up without an adult behind the bus to guide the driver. Since most school buses have no second party on board, and there is always a chance that a car will be parked on both sides of the cul, we (planning Board) require a 55 foot radius paved cul. Since the RSIS permits less, we will permit the lesser radius , with the proviso that all sales maps state promanantly that school buses will not be permitted down that cul de sac. The developer will often volunteer a 55 foot radius.

Trafficpro

 

[nickinnj]

The RSIS governs in NJ as Trafficpro states. The ADT is 250 trips. Using the factor of 10.1 trips per single family home limits the total lots to 24. The length is dictated by the required zoning of the muncipality. If you want a copy the charge is $10.00 and write to New Jersey Department of Community Affairs, Publications Unit, P.O. Box 802 Trenton N.J. 08625-0802. Also read Al Pagan's article in CENews Janurary 2003 edition.
Not all planning boards require a 55 foot paved cul de sac as suggested. A muncipal ordiance prohibiting parking may reduce the concern. The minimum 40 ft. radius as required in the RSIS has been the standard in many towns for a long time and works quite well.However that is an issue between the developer and the board.

 

[KRSServices]

I believe the length has been developed in two ways. First of all, many municipalities of similar stature and size will apply a standard if it is working well in other communities, particularly if the standard has been in place for a number of years. It's kind of like the why re-invent the wheel concept. Secondly, I know that traffic flows begin to get "busy" from a residential perspective and those long dead-end streets become more of a nuisance. Many communities I have had the pleasure to work for tend to design cul-de-sacs with no more than 10 to 20 lots. All waterlines are looped and they seem to sell incredibly faster than longer winding street frontages. Just be careful to ensure the bulb radius is sufficient for complete turning of emergency vehicles (with or without street parking) and that the hydrants are spaced properly (150 m or so). KRS Services

http://www.krs-services.com

 

[jrfosterjr]

Here are a few thoughts about what we do in our area (NC):

1. With much success, we've built cul-de-sac roads of several thousand feet with one caveat, it must be privately owned and maintained by the development's HOA.
2. Emergency access is certainly an issue, so we often incorporate loops along the course of the cul-de-sac, using the open space for tree planting and other visual effect. This provides a bypass in the event of an accident and some variation from repetition of house after house after house.
3. To slow down traffic and add visual feature, we incorporate traffic circles.
4. Looping of waterlines is a must to ensure access to water for firefighting purposes and supply in case of a break.
5. One thing we are doing in a current project is building a paved alley, if you want to call it that, from one cul-de-sac bulb to another. We put about a 25' separation between two lots to accomodate this paved strip. It is blocked for regular traffic, but accessible by emergency and maintenance vehicles. Local residents would have to unlatch a gate and jump the curb to gain access, making it unfeasible for normal use.
6. For publicly owned cul-de-sac streets, we have similar limitations to those named by others with some extension allowed due to terrain or other site limitations such as triangulated land parcels, major stream or wetland crossings, etc.

Hope this is helpful. Jeff Foster, PE
CE Group, Inc.
Apex, NC

 

[washark]

What about offset cul-de-sac? Is there a minimum/maximum distance of offset? Can it be any anywhere between 0 and whatever half of you road width?

 

[trafficPro]

We find the best flow of traffic occurs when the cul is tangent to the right (incoming)side of the road. In other cases we increase the radius of the curb return where the cul bulb meets the roadway, to provide better flow. The radius depends on the alignment of the road and bulb. Throw a curve template on it, or use Autoturn.
TrafficPro

 

[washark]

So you have seen an offset that is not completely tangent to the either the incomming or outgoing side?

 

[trafficPro]

Many culs have lolly pop alignment. That is the cul is centered on the incoming roadway. The criteria seems to be the ability of the developer to get a reasonable lot layout.
Once the roadway is located make sure the radii are sufficient going in and out. Sometimes a central landscape island is provided to get larger turn radii. Thjis is acceptable ONLY in privately owned developments, as the island need uipkeep.

 

[trafficPro]

Many culs have lolly pop alignment. That is the cul is centered on the incoming roadway. The criteria seems to be the ability of the developer to get a reasonable lot layout.
Once the roadway is located make sure the radii are sufficient going in and out. Sometimes a central landscape island is provided to get larger turn radii. This is acceptable ONLY in privately owned developments, as the island need upkeep.