Variable speed limits: Evaluation of the road weather controlled section

Trials with Variable Speed Limits (VSL) for The sections differ from each other. The site different applications are carried out by the in Halland is a fairly traffic intense highway Swedish Road Administration (SRA). The (18000 vehicles/day) while Blekinge is a three goal is to demonstrate if and how VSL can lane highway with centre crash barrier (2+1) contribute to a better speed adaptation in a and average daily traffic of approximately cost-efficient way. Road weather controlled 8500 vehicles/day. sections are one application area. For this type of application the maximum allowable The evaluation programme involves the fol- speed limit is adjusted downwards to diffe- lowing main activities: rent levels when adverse weather and road • Follow-up analyses of operational data surface conditions occur. Preliminary results • Measurements and analyses of traffic data from the first trial period are presented in (traffic counts, speed- and performance this report. studies) • Driver attitude surveys (questionnaires in Road weather stations (RWIS) at the trial si- Blekinge and Halland) tes collect weather data from the air as well • Cost-benefit analyses (including impact as above and under the road surface. These assessments regarding traffic safety, tra- data are processed by a weather model at vel time and environment) the SRA Head Quarters from where alerts • Scanning (international experiences, etc) signals are sent to the regional Traffic infor- mation centre (TIC). Operators activate the Operational follow-up Variable speed limit signs (VSL signs) with The technical equipment functioned satis- shining white figures in a red ring on dark factorily except for the communication sys- background resulting in a change of the dis- tem in Blekinge. Due to the problems in Ble- played speed limit. Every step is logged and kinge the accessibility measure, meaning the the information is transmitted to a central share of time when all VSL signs could be data-base. correctly controlled at adverse weather con- ditions, was only registered to 62%. In Hal- The trial sites included in the report are a 17 land however the technical km section of E22 in the province ofBlekinge accessibility measure was 99.2 %. Due to (Åryd-Ronneby), 55 km of E6 in Halland wind sensor problems at the Uddevalla site, (Skottorp–Heberg) and the Uddevalla Bridge. incorrect values were registered. This sensor The first mentioned two establishments are is now disconnected. controlled based on the road surface condi- The sign control procedure did not work ful- tion and the Uddevalla Bridge is controlled ly satisfactorily. There are quality drawbacks based on crosswind. in the operator support systems (such as the weather model, RWIS-data etc). ploughing and salting mission. This might take long, especially at night, before the contractor has moved to the site. During that time the speed limit might be insuffi- Since data from RWIS are delivered in 15 cientlyThe decreased increase by the operator although of thethe conditions maximum call for lower limits. permitted This might give the drivers a false message. Improvements are needed. minutes cycles, alerts from the weather mo- speed limit in Blekinge from 90 to 110 km/h Traffic impacts del arrive late and there could be big jumps Trafficmade data were collected the average in pre and pos t studies speed carried to out during increase two consecutive by 4-5 winter seasons. A second follow-up was accomplished in Blekinge a year later to in suggested levels. Contacts with the road observekm/h. the long term This effects. is also valid for Uddevalla alt- maintenance contractor are needed for con- The speedhough adaptation there was clearly was improved no by raisethe VSL system of theat both speedsites (Halland limit and Blekinge). This is especially true for passenger cars. The most evident impacts firmation of indicated severe road surface occur there.at low speed Thuslimits. The average the speed observance is then 15-20 km/h of below the the level highest that the drivers choose during corresponding adverse conditions with traditional fixed conditions (levels 80 and 60 km/h) and for signs.speed See figure below. limit increased. In Blekinge the ob- information about completed ploughing and The averageservance speed in Halland of increased the speed only by 2 km/h limits during good also conditions improved salting mission. This might take long, espe- despitewhen the fact that wet the highest road speed limit surface was changed or from snow 110 to 120 (displayedkm/h. One might speculate why it did not increase more. One reason could be that it is unclear to cially at night, before the contractor has mo- the drivers100 where and the VSL90 systemkm/h starts respectively). and ends. Another explanation could be that they consider the speed limit around 110 km/h to be comfortable. ved to the site. During that time the speed The increase of the maximum permitted speed limit in Blekinge from 90 to 110 km/h limit might be insufficiently decreased by the made Howeverthe average speed theto increase observance by 4-5 km/h. This isof also the valid forlowest Uddevalla speed although there was no raise of the speed limit there. Thus the observance of the operator although the conditions call for lo- highestlimits speed limit at increased. severe In Blekinge road the observance surface of the speedconditions limits also de- improved when wet road surface or snow (displayed 100 and 90 km/h respectively). wer limits. This might give the drivers a false Howevercreased the observance as of expected. the lowest speed limits at severe road surface conditions message. Improvements are needed. decreased as expected. 120 120 Halland before Halland after Blekinge before Blekinge after Traffic impacts 110 110 110 110 110 110 110 110 Improved speed Traffic data were collected in pre and post adaption with VSL, studies carried out during two consecutive 100 100 100 14 km/h and 20 km/h winter seasons. A second follow-up was ac- 90 90 90 90 90 90 90 complished in Blekinge a year later to obser- ve the long term effects. The speed adapta- 80 80 80 tion was clearly improved by the VSL system 70 at both sites (Halland and Blekinge). This is 60 60 60 especially true for passenger cars. The most Dry surface Wet surface Snow Ice Severe ice evident impacts occur at low speed limits. Figure: Average speeds at Hökaslätt in Halland and Bräkne-Hoby in Blekinge (both directions) at Figure:different road Average surface conditions speeds with and without at VSL Hökaslätt in Halland The average speed is then 15-20 km/h below and Bräkne-Hoby in 80Blekinge (both directions) the level that the drivers choose during cor- at different road surface conditions with and responding adverse conditions with traditio- without VSL nal fixed signs. See figure below. It can be concluded that the VSL system gives The average speed in Halland increased only a minor contribution to better speed adapta- by 2 km/h during good conditions despite tion at fairly difficult conditions. At hard and the fact that the highest speed limit was very rough conditions (snow, ice, and severe changed from 110 to 120 km/h. One might ice) the VSL system gives significant extra speculate why it did not increase more. One stimuli to the drivers to adjust their speed reason could be that it is unclear to the dri- properly. vers where the VSL system starts and ends. Another explanation could be that they con- Long time measurements were conducted sider the speed limit around 110 km/h to be in Blekinge one year after the first measu- comfortable. rement period with the system operational. The results show a continuing decrease of database STRADA state that the number of the average speed to an even lower level than accidents with injuries increased from 4 to during the first follow-up period. This is es- 13 during the first three years and that the pecially evident when 60 km/h is displayed. number of fatalities and severely injured in- creased from 1 to 2 individuals after the in- Attitudes troduction of a raised maximum permitted The drivers´ respect of the speed limits speed limit and VSL. However the follow-up strengthened with the introduction of VSL. periods are far too short. Thus it is important In total, more than 90 % of the respondents to continue the accident analyses over longer in Halland and about 70 % in Blekinge consi- periods of time. der the system to be good. The most frequent comment to why the system is good is that Performance it contributes to a more harmonised traffic The idea behind weather controlled variable flow. About 80 % of the drivers believe that speed limits is to be able to accept higher they have become more attentive regarding speeds at good conditions (dry surface) as a the road surface when passing an activa- trade off for differentiated speed regulations ted VSL sign. A majority of the respondents during periods with adverse weather and in both surveys also believes that display- road surface conditions. In Halland the time ed speed limits are in line with the current consumption decreased by 1.3 % while using weather and road surface conditions while VSL in combination with a raised maximum about one third disagree to this. Almost half speed limit. The corresponding figure for Ble- of the drivers in Blekinge thus consider the kinge is 1.4 %. Thus the impact on time con- signs sometimes being out of order or that sumption is minor. the wrong speed message is displayed. Re- gardless of the reason behind this opinion, it Environment is important to display speed limits that are The environmental impact from VSL is mar- apprehended as correctly as possible with ginal. Raising the maximum speed limit in regard to current road surface conditions. Halland from 110 to 120 km/h resulted in 0.6 Otherwise it will be difficult to maintain pu- % increase of carbon dioxide emissions. This blic credibility. is however partly compensated by a decrease in emissions during periods of lower speeds The drivers believe that observance and at adverse weather conditions. The resulting speed adaptation can be increased by more impact for Halland is 0.4 % increase. A con- speed enforcement. However to an even hig- clusion is that VSL solely influence the envi- her degree they think that information about ronment positively although to a negligible the reasons for decreasing the speed limits degree. leads to better attention and observance. In Blekinge the increase of carbon dioxi- de emissions is 2 % caused by the raise of Traffic safety the maximum speed limit from 90 to 110 Traffic safety, represented as difference in km/h just before the introduction of the VSL accident figures, was calculated using the system. traditional power model based on measured speeds and traffic load during different road Socio economic impacts surface conditions. The Halland study re- The socio economical calculations for Hal- vealed no difference with regard to the total land reveal a benefit cost ratio of +1.6. Thus number of accidents. However the number of the investment is beneficial provided that fatalities and severely injured increased by correct speed limits are displayed during 80 4% (at 80 % functional accessibility). Statis- % of the time when adverse weather condi- tics shows 11 people killed or severely inju- tions occur. This accessibility level however red before the introduction of VSL. After the is not sufficient to make the investment in introduction the number of fatalities and Blekinge beneficial (benefit cost ratio +0.8). severely injured decreased from 22 to 15 in- A sensitivity analysis shows that the acces- dividuals during the first two years. This ten- sibility level for correct speed limit display dency is promising. that adverse weather conditions need to be prevailing for 92 % of time in order to obtain For the Blekinge case figures from the accident the same benefit ratio as in Finland (+1.9). Analyses of different isolated measures for - The lowest displayed speed limits should Blekinge indicate that raising the maximum be activated directly upon alarm after a speed limit from 90 to 110 km/h alone results short check of the RWIS data, meteorolo- in significant negative safety consequences. gical prognosis and possibly cameras. If However the introduction of VSL, when the needed, an adjustment of the displayed maximum speed limit is 110 km/h, is a very speed limit may be made based on contact profitable measure (benefit cost ratio +7.6). with the maintenance crew The analysis also indicates that a decrease - New road surface sensors need to be suf- of the maximum permitted speed limit to ficiently tested and validated using friction 100 km/h in combination with a reduction of measurement equipment during actual the number of alert or speed display levels conditions of the VSL system to three levels, which has been suggested, will become a very valuable On a long time span: measure. - Introduce a common principle for variable speed limits on weather controlled road Suggested action sections. Three levels are suggested (good, To obtain profitability, better accessibility is severe and very severe driving conditions) necessary especially in Blekinge, which me- in 20 km/h steps and the lowest at 50 km/h. ans that the part of time when correct speed - Adjust the criteria to this three level prin- is displayed during adverse conditions must ciple also for sections with wind force reach a higher level than measured during control. the trials. Improved profitability - Introduce fully automatic control (monito- might also be received by reducing and ad- red from Traffic Information Centre) justing the displayed speed levels. This imp- lies that steps must be taken to improve the In order to achieve even better speed adap- technical performance and the operational tation to the prevailing road surface con- handling of the system. ditions the following measures should be considered: In a short time perspective the following ac- - Conduct frequent speed checks. Develop tions might be relevant: automatic enforcement systems to handle - Improve the communications system in variable speed limits. Average speed enfor- Blekinge. More robust technique need to be cement systems (section control) might be installed suitable for weather controlled VSL. - Improve the data delivery from RWIS. Data - Introduce dynamic information about the must be requested more frequently than reason for a lower speed limit. This is im- today (15 minutes) in order to react upon portant at situations when surface condi- sudden weather changes like heavy sum- tions are difficult for the driver to detect mer rainfalls (black ice etc) but it is also essential for - Improve the support systems of the TIC. systems controlled by more than one type The VSL routines need to be integrated in of condition (weather, traffic etc)

the normal work instructions ORDER NO. 100377. PRODUCTION: VÄGVERKET. VÄGVERKET.

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