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HERS-ST Discussion
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Messages posted for Speed Limits (Item 63)
Subject: Speed Limits (Item 63)
From:
Rich Arnold
Date: 02/04/2003 |
| For those of us who build HPMS type datasets, but use more detailed data than what is required by the submittal
.. By HPMS definition, the user is asked to enter the daytime speed limit for automobiles. The rural interstate system in Oregon is posted with two speed limits, one for autos (65mph) and the other for trucks (55mph). The differential between speeds is most noticeable for rural segments near urban MPO regions, where the number of trucks filling the roadway seems to increase the most. Congestion analysis accounts for the percentage of trucks on the road, but it doesnt seem to account for the speed reduction associated with the posted speed. In other words, not only are there more trucks on the roadway, but they are going slower than the autos because of regulations. Subsequently, there is increased congestion associated with this dilemma. Any thoughts on how to account for this in the HERS analysis??? |
Subject: RE: Speed Limits (Item 63)
From:
Rich Margiotta
Date: 02/05/2003 |
| This is an excellent point. Neither the revised HPMS capacity procedures nor the HCM discuss this effect. Basically, speed limit is one of the factors that influence a section's free flow speed (FFS), although the extent to which it does is somewhat contentious and the HCM stays away from the issue (for freeways, anyway). As FFS decreases, so does capacity (HCM, p. 23-5). In theory, dual speed limits should lower the FFS over what one normally assumes, but how the two speed limits are blended is a problem and I haven't seen any discussion of how to do it. One could envision an adjustment to the base free flow speed in the new capacity procedures, but exactly what that adjustment should be is anybody's guess. This is probably best dealt with by the HCM committee since it has broader implications than just for HPMS and HERS. --Rich |
Subject: RE: Speed Limits (Item 63)
From:
Rich Arnold
Date: 10/28/2003 |
| Oops, the Posted Speed Limit is Item 80 for the latest HPMS format. An important question to ask here is how does HERS use the posted speed entry in calculations? The HPMS Field Manual states that it is used in investment requirements modeling to estimate running speed and for other analysis purposes, including delay estimation (page IV-32). The user is instructed to enter the daytime speed limit for automobiles posted or legally mandated, and if there is no legally mandated maximum daytime speed limit for automobiles, code 999. This leads to the primary question on how this item is used for calculations within the model, since I would assume that there would be a different response if the item were coded 999 rather than 055 or 035. The posted speed limit for autos and trucks on Oregons rural interstate system is 65 mph and 55 mph, respectively. At times, the disparity in speeds leads to a notable congestion issue, specifically on four lane roadway systems (two lanes per direction) when the truck percentage is high. In these situations, the outside lane contains the slower truck traffic and all the automobiles have subsequently moved to the inside lane in an attempt to pass the slower truck traffic. In essence, as the truck traffic increases, the system begins to resemble an average two-lane highway (one lane in each direction) for the auto traffic. Subsequently, the auto speed tends to be closer to the posted truck speed when a large percentage of trucks are prevalent. Which leads me to code the posted speed limit at 55 mph for four lane roadways with heavy trucks, even though the actual posted speed for automobiles is 65 mph (as defined in Item 80 HPMS Field Manual). This leads back to the question on how this item is used for calculations within HERS, and how much of a difference it make in the analysis whether the item is code 55 mph vs. 65 mph. Enlightenment???? |
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